Aaron Alexis, believed to be the lone gunman in the killing of 12 people at a Navy building in Washington Monday morning, was a Navy veteran from New York with a troubled past who was arrested in two previous shooting incidents. 
 
 Alexis, 34, who was discharged from the Navy two years ago after serving hitches in Texas and Illinois, sprayed bullets from the fourth floor down to the cafeteria area in the Naval Sea Systems Command headquarters in southeast Washington in an attack that began just after 8 a.m. Alexis died later as he traded shots with responding police, though it was not clear if he killed himself or was brought down by cops. 
 
 At a late night news conference, Metropolitan Police Chief Cathy Lanier said she was confident that Alexis was the "single and sole person responsible" despite earlier reports of a possible second shooter. 
 
 Washington Mayor Vincent Gray said that there was still no motive for the shootings and no indication of terrorism "although we haven't ruled that out." 
 
 Hewlett Packard issued a statement Monday night saying that at the time of the shootings, Alexis worked for The Experts, a subcontractor on an HP Enterprise Services contract to refresh equipment used on the Navy Marine Corps Intranet network. The FBI said he had legitimate access to the Navy Yard "as a result of his work as a contractor." 
 
 Gray said those killed ranged in age from 46 to 73. In addition, he said eight people were injured -- three by gunshots and the other five by non-shooting causes like stress and falls. 
 
 Late Monday night, D.C. police identified seven of the 12 victims: Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61. 
 
 Two federal officials told the Associated Press that Alexis had an AR-15 assault rifle, a shotgun and a handgun that he took from a police officer at the scene. The two officials spoke on the condition of anonymity because they were not authorized to discuss a pending investigation. 
 
 Many who managed to escape in the early minutes of the episode recalled panic and fear after a routine Monday morning was shattered by gunfire. 
 
 "They sounded like 'pop, pop, pop,'" said Patricia Ward, a logistics management specialist who was in the cafeteria. "Everybody just panicked at first ... It was just people running, running, running. 
 
 "I just kept running," Ward said. "Our mission is to take care of the Navy ... After today, it's not secure enough for me." 
 
 Other witnesses told similar stories of gunshots and chaos. 
 
 “A little after 8, we heard a loud noise and didn’t think anything of it,” U.S. Navy Capt. Mark Vandroff told Fox News. “Someone with the presence of mind, and if I find out who it is, I’m going to thank them for it, closed our office doors. 
 
 “We got down on the floor as low as we could and barricaded ourselves in with tables and chairs,” he added. Vandroff said he frantically texted co-workers to check on them until police led them out of the office around 10 a.m. 
 
 Throughout the morning and early afternoon, helicopters circled above the sprawling complex. People still trapped inside the building, where some 3,000 work every day, were told to "shelter in place," as helicopters dropped baskets to pick up the wounded. 
 
 Alexis, 34, of New York, was a Navy aviation electrician's mate, third class, who spent time in Texas and Illinois before leaving the Navy in early 2011. It was not clear why he left or if he was voluntarily discharged. A senior defense official told Fox News he had a "history of misconduct." 
 
 While stationed in Fort Worth, he was arrested on Sept. 4, 2010, after an upstairs neighbor reported that he had fired shots up into her apartment.The neighbor said Alexis had often angrily confronted her about making too much noise, and she told police he scared her. 
 
 "[The neighbor] told me that she is terrified of Aaron and feels that this was done intentionally," the arresting officer wrote on the police report. 
 
 In 2004 he was arrested in Seattle for shooting out the tires of a construction worker's car in an incident he later described as "an anger-fueled blackout." 
 
 He also told Seattle police he had witnessed the Sep. 11, 2001 terrorist attack and "how these events had disturbed him," according to a police report. 
 
 Alexis' father told police in 2004 his son had participated in rescue efforts during 9/11 and had suffered post traumatic stress disorder. This could not be independently confirmed. 
 
 A former neighbor of Alexis' family in Queens, N.Y. told Fox News, "I'm shocked and appalled. My heart goes out to those families. I guess you never know who lives next to you." 
 
 Lanier said the first call came in just after 8:15 a.m., and that officers were on the scene within minutes. She said one member of her department was shot after engaging with a shooter, who she said was killed. Lanier initially said there were "potentially two other shooters" at large, although one white male who was being sought was later cleared. 
 
 The incident paralyzed much of the city, with several nearby schools going on lockdown. The Washington Nationals, who play near the scene, postponed their night baseball game. 
 
 Gray said investigators had not identified a motive for the shooting. There was no indication that the incident was an act of terrorism, he said. The FBI was asking the public to provide any information that might help them learn more about Alexis's possible motive by calling (800) CALL-FBI or by clicking here. 
 
 Rick Mason, a program management analyst who is a civilian with the U.S. Navy, said a gunman was shooting from a fourth floor overlook in the hallway outside his office. Mason said the gunman was aiming down at people in the building's cafeteria on the first floor, adding that he could hear the shots but could not see a gunman. 
 
 Shortly after the gunfire, Mason said overhead speakers told workers to seek shelter and later to head for the gates at the complex. 
 
 People who were inside the building said a gunman wordlessly sprayed bullets as terrified civilians and Navy members scattered. 
 
 "We saw him hold the rifle, and we saw him aim it in our direction," a witness told FoxNews.com. 
 
 Another witness told WJLA: "We were looking, but he was down the hall far enough that we couldn't see a face. But we saw him hold the rifle and then we saw him raise it and aim in our direction." 
 
 U.S. Navy Secretary Ray Mabus said two Navy employees and one Metro DC officer were brought to Medstar Washington Hospital Center, not with injuries considered life threatening. 
 
 Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. It builds, buys and maintains the Navy's ships and submarines and their combat systems. 
 
 White House officials, in a statement released shortly after the shooting, confirmed that President Obama had been briefed on the incident. 
 
 "The President directed his team to stay in touch with our federal partners, including the Navy and FBI, as well as the local officials," the statement read. "We urge citizens to listen to the authorities and follow directions from the first responders on site." 
 
 Obama later promised to make sure those responsible for the "cowardly act" are held accountable. 
 
 More than 20 members of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) responded to the scene, including the same Special Response Team that extracted the alleged Boston Marathon bomber Dzhokhar Tsarnaev from the boat where he barricaded himself following the April 15 attack. 
 
 Although the motive was not yet known, the shooting brought to mind another mass shooting at a military base, on Nov. 5, 2009. That shooting, carried out by Army Maj. Nidal Hasan, an American-born Muslim who became a jihadist while in the military, left 13 dead and more than 30 injured. Hasan was convicted of multiple counts of murder on Aug.23 and sentenced to death by a military judge five days later. 
 
 The Associated Press contributed to this report. ||||| WASHINGTON — A former Navy reservist killed at least 12 people on Monday in a mass shooting at a secure military facility that led the authorities to lock down part of the nation’s capital — even after the gunman was killed — in a hunt for two other armed men spotted by video cameras, officials said. 
 
 But by Monday evening, the federal authorities said they believed the shooting was the act of a lone gunman, identified as Aaron Alexis, 34, who was working for a military subcontractor. 
 
 The chaos at the facility, the Washington Navy Yard, started just after 8 a.m. Civilian employees described a scene of confusion as shots erupted through the hallways of the Naval Sea Systems Command headquarters, on the banks of the Anacostia River a few miles from the White House and about a half-mile from the Capitol. 
 
 “I heard three gunshots, pow, pow, pow, straight in a row,” said Patricia Ward, a logistics management specialist from Woodbridge, Va., who was in the cafeteria on the first floor when the shooting started. “About three seconds later, there were four more gunshots, and all of the people in the cafeteria were panicking, trying to figure out which way we were going to run out.” 
 
 Police officers who swarmed the military facility exchanged fire with Mr. Alexis, 34, a former naval reservist in Fort Worth. Police officers shot Mr. Alexis to death, law enforcement officials said, but not before a dozen people were killed and several others, including a city police officer, were wounded and taken to local hospitals. 
 
 Officials said Mr. Alexis drove a rental car to the base and entered using his access as a contractor and shot an officer and one other person outside Building 197, the Sea Systems Command headquarters. Inside, Mr. Alexis made his way to a floor overlooking an atrium and took aim at employees eating breakfast below. 
 
 “He was shooting down from above the people,” one law enforcement official said. “That is where he does most of his damage.” 
 
 The names of seven of the victims were released late Monday: Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61. Officials said names of the other victims would be released after their families had been contacted. All of the victims were believed to be civilians or contractors. No active duty military personnel were killed, said Chief Cathy L. Lanier of Washington. 
 
 One victim was shot in the left temple and was pronounced dead within a minute of arriving at George Washington University Hospital. “This injury was not survivable by any stretch,” a hospital official told reporters. “The patient was dead on the way to the hospital.” 
 
 Eight people were injured. Three of them were shot, including Officer Scott Williams of the Washington police. The others suffered injuries from falls or complained of chest pains. Officer Williams, who served in the canine unit, underwent several hours of surgery for gunshot wounds to his legs. A second victim suffered a gunshot wound to her shoulder. A bullet grazed a third victim’s head but did not penetrate her skull, according to doctors at MedStar Washington Hospital Center. 
 
 Three weapons were found on Mr. Alexis: an AR-15 assault rifle, a shotgun and a semiautomatic pistol, a senior law enforcement officer said. It was unclear whether he had brought all the guns with him, another law enforcement official said, or if he had taken one or more of them from his victims. 
 
 Officials said they were still searching for a motive as they asked the public for help by posting pictures of Mr. Alexis on the F.B.I. Web site. The agency is treating the shooting as a criminal investigation, not one related to terrorism. 
 
 Navy officials said late Monday that Mr. Alexis had worked as a contractor in information technology. A spokesman for Hewlett-Packard said Mr. Alexis had been an employee of a company called The Experts, a subcontractor on an HP Enterprise Services contract. 
 
 Reporting was contributed by Abby Goodnough, Emmarie Huetteman, Thom Shanker, Sarah Maslin Nir and Joseph Goldstein from Washington, and William K. Rashbaum from New York. ||||| A gunman killed 12 people and injured eight inside a heavily secured building at the Washington, D.C., Navy Yard Monday, in one of the worst mass shootings ever at a U.S. military facility, and one of the deadliest single events ever in the nation's capital. 
 
 The suspected gunman, 34-year-old Aaron Alexis of Fort Worth, Texas, was also killed. Alexis had served as a Petty Officer in the Navy between 2007 and 2011, and had been previously arrested in gun-related incidents. 
 
 UPDATE: Click here for the latest on the Navy Yard Shooting 
 
 Those killed ranged in age from 46 to 73, according to D.C. Police. Officials released the following victims' names Monday night: 
 
 FBI Urges Public to Provide Details About Navy Yard Shooter 
 
 Mayor Vincent Gray, Police Chief Cathy Lanier, Valerie Parlave of the FBI and Congresswoman Elanor Holmes Norton brief press about ongoing search for potential 2nd shooter, additional death reports bringing total death toll to 13, and the FBI urges the public to provide details about known shooter, Aaron Alexis. (Published Monday, Sept. 16, 2013) 
 
 Michael Arnold, 59 
 
 Sylvia Frasier, 53 
 
 Kathy Gaarde, 62 
 
 John Roger Johnson, 73 
 
 Frank Kohler, 50 
 
 Kenneth Bernard Proctor, 46 
 
 Vishnu Pandit, 61 
 
 D.C. Mayor Vincent Gray reported late Monday eight individuals were injured, including a D.C. police officer, identified as Scott Williams, who was shot in the leg while responding to the shooting. After being in surgery for hours, doctors say they hope he will be able to walk again. 
 
 None of the fatalities is reported to be military personnel. The rest of the injured individuals suffered non-life threatening injuries and are expected to recover. 
 
 Also Monday night, FBI and ATF agents searched a D.C. hotel where Alexis, who recently began work as a civilian contractor, was reportedly staying with five co-workers in town for business. 
 
 Alexis allegedly opened fire just before 8:30 a.m. Monday along the third and fourth floors of Building 197 of the Naval Sea Systems Command Headquarters in Southeast D.C. Bullets hit walls, and people around them fell to the floor, bleeding. Officials reported Alexis had "legitimate access" to enter the Navy Yard at the time as a civilian contractor, using a valid pass. 
 
 Survivors of the morning attack recalled sprinting from the sound of gunfire on the third and fourth floors of Building 197 of the Naval Sea Systems Command Headquarters. Bullets hit walls, and people around them fell to the floor, bleeding. 
 
 13 Dead, at Least 3 Wounded in Navy Yard Shooting 
 
 Thirteen are dead and at least three others are recovering from gunshot wounds after a shooting at Navy Yard Monday morning. News4's Pat Collins reports. (Published Monday, Sept. 16, 2013) 
 
 "I was in the cafeteria, just paid for my breakfast," Patricia Ward said. "I was waiting for my friend to pay for hers when we heard the gunshot. It was three gunshots straight in a row, pop pop pop. Three seconds later, it was pop pop pop pop pop, so it was like about a total of seven gunshots, and we just started running." 
 
 When it was over, the suspected gunman lay dead amid an armload of weapons. Sources told News4 that surveillance footage showed that he began his attack with a shotgun, but was found with a 9mm pistol. Police confirmed that there was no AR-15 assault rifle used, as previously reported. 
 
 NBC News correspondent Pete Williams is reporting Alexis purchased a shotgun in Lorton, Va. during the past week or so. 
 
 Dramatic Photos: DC Navy Yard Shooting 
 
 The suspected gunman appeared to have seized firearms from two of his victims as he moved through the building along the Anacostia River in southeast Washington, where 3,000 Navy employees go to work each day, many of them carrying authorized firearms. 
 
 Next to Alexis's body was an ID of a man who used to work at the Navy Yard. Some investigators believe the suspected gunman used that ID to enter the area, which is accessible only through guarded gates, but he may also have shot his way in, Williams reported. 
 
 Obama: "We Are Facing Another Mass Shooting" 
 
 President Barack Obama addresses the multiple shooting at Navy Yard. (Published Tuesday, Sept. 17, 2013) 
 
 The pandemonium continued after the suspected gunman was killed, as authorities suspected that more than one gunman remained on the loose. Eight nearby schools were put on lockdown. And as of Monday evening, D.C.'s mayor still said investigators are examining whether there was another gunman at large. 
 
 Terrie Durham, a civilian employee in the Office of Naval Sea Systems Command, was looking for a way out and glimpsed the gunman down a hallway, about 40 feet away. He was wearing blue, a uniform, maybe, and held a rifle. He said nothing. Then he fired. 
 
 "He aimed high and missed," Durham said. "I saw where the shot missed. It was a few feet ahead of us and a foot or above where we would have been standing." 
 
 Mayor, Police Chief Speak About Navy Yard Shooting 
 
 Police say as many as two gunmen may still be at large near the D.C. Navy Yard Monday after a shooting that began in a heavily secured building and left a number of people dead. One other gunman has been killed, Metropolitan Police Chief Cathy Lanier said at a midday press conference. (Published Monday, Sept. 16, 2013) 
 
 She took off into a stairwell, hollering, "There is a man shooting in the building, get out, get out!" 
 
 Multiple weapons 
 
 Survellance video shows the gunman entered the NAVSEA building, at 1336 Isaac Hull Ave., with a shotgun, law enforcement officials told News4's Jackie Bensen. 
 
 Navy Yard Shooting: Pentagon Correspondent Reports 
 
 Four people were killed and 10 were shot - two of the victims police officers - at the Washington, D.C. Navy Yard Monday morning, and the shooter has not yet been apprehended, authorities confirmed. NBC"s chief Pentagon correspondent Jim Miklaszewski has the latest on the shooting. (Published Tuesday, Sept. 24, 2013) 
 
 The suspected gunman shot a security officer in the head, killing him, and took the officer's 9 mm pistol and a magazine of ammunition. The shooter then continued through the building and seemed to target his victims, who were mostly on the third and fourth floors, Bensen reported. 
 
 D.C.'s Metropolitan Police Department and several other law enforcement agencies responded with active shooter teams. 
 
 During that response, a MPD officer was shot in the leg. The gunman was then shot by a FBI hostage response team, law enforcement sources told Bensen. 
 
 Navy Yard Shooting: Early Report 
 
 The U.S. Navy said the shooting began at about 8:20 a.m. with three shots fired inside Building #197 at the Naval Sea Systems Command Headquarters. U.S. military officials say the gunman first shot the base security officer. (Published Tuesday, Sept. 17, 2013) 
 
 “The response by uniformed police officers from both the Metropolitan Police Department and United States Park Police was absolutely nothing short of heroic,” MPD Chief Cathy Lanier said. 
 
 Authorities are investigating whether the gunman took the D.C. police officers' rifle. 
 
 D.C. Police had said earlier Monday that they were seeking two other "possible" gunmen. Investigators later ruled one of them out as a suspect by watching video that tracked the man through the building and realizing that no harm resulted from his movements. 
 
 Williams noted that many people are authorized to carry firearms at Navy Yard. 
 
 However, at an afternoon press conference, Mayor Vincent Gray said police were still working to rule out another suspect. Police released a lookout for a "possible" second shooter, described as a man between 40 and 50 years old, who was wearing olive drab and was armed with a "long gun." He was about 5-feet-10-inches tall and 180 pounds, African-American, with a medium complexion and graying sideburns. 
 
 That lookout was canceled late Monday evening and D.C. Police said they felt comfortable they had the "single and sole person responsible." 
 
 The motive is unknown, Gray said, though there were reports that Alexis may have had a grudge against the Navy. Gray said while there is no reason to suspect terrorism, it can't be ruled out yet. 
 
 The FBI now is leading the investigation into what is the deadliest single incident within D.C. borders since 1982, when Air Florida Flight 90 crashed, killing 78 people. 
 
 Obama: 'They're patriots' 
 
 “We do know that several people have been shot, and some have been killed,” President Obama said from the White House Monday. “So we are confronting yet another mass shooting, and today it happened on a military installation in our nation’s capital. It’s a shooting that targeted military and civilian personnel. 
 
 "These are men and women going to work, doing their job protecting all of us," Obama said. They’re patriots, and they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn’t have expected here at home." 
 
 NAVSEA is the largest of the Navy's five system commands. The Washington Navy Yard will be open only to mission essential personnel Tuesday, as investigation continues. 
 
 Though Metropolitan Police Chief Cathy Lanier stressed that the scene was contained, people near the base were asked to stay in their homes, and eight D.C. public schools and one private school nearby were locked down. 
 
 A large search, including from helicopters, continued into Monday afternoon. "It's a large piece of land with many buildings," Assistant D.C. Police Chief Peter Newsham told News4. "It's going to take a while to determine that it is safe." 
 
 Earlier Monday, tactical teams were seen entering at least two buildings on the Navy Base to search for the shooter, and News4's Mark Segraves reported that three helicopters were seen tightly circling part of the base. 
 
 Treating the victims 
 
 MedStar Washington Hospital Center said the MPD officer was among three victims that had been transported to that hospital. The officer was shot in the legs; one other victim was shot in the shoulder and one was shot in the head and hand. 
 
 All are alert and speaking and expected to make full recoveries, said Dr. Janis Orlowski, chief medical officer at the hospital. All are in critical and stable condition. 
 
 The police officer is out of surgery. The female Navy employee shot in the shoulder also required surgery. The other woman who works for the Navy did not require surgery. The gunshot to her head did not penetrate the skull. 
 
 "There's something evil in our society that we as Americans have to work to try and eradicate," Orlowski said. "I have to say, I may see this everyday, I may be the chief medical officer of a very large trauma center, but there's something wrong when we have these multiple shootings, these multiple injuries. There's something wrong. The only thing that I can say is we have to work together to get rid of it." 
 
 The hospital was prepared to accept more victims, though it does not appear that will be necessary. 
 
 George Washington University Hospital said they had also received a victim, a man in his 60s who had been shot in the temple. Despite efforts at CPR, he was prounounced dead at the hospital. 
 
 One victim was evacuated from the roof of a building, reported News4's Tony Tull. A U.S. Park Police helicopter hovering above the scene around 9:50 a.m. lowered a basket to a building and lifted what appeared to be a shooting victim from the roof. The helicopter came back to the scene just after 10 a.m. to retrieve another victim. 
 
 Anyone with information about this incident should call 1-800-CALL-FBI. The FBI wants to talk to anyone who knew Alexis. No piece of information regarding his recent movements, contacts and associates is too small. 
 
 Family members seeking information about loved ones should call 202-433-6151 or 202-433-9713. 
 
 Stay with NBCWashington on-air and online for more on this developing story. 
 
 MORE COVERAGE ON NBCWASHINGTON.COM:
– A picture of alleged Navy Yard shooter Aaron Alexis, who was apparently a military contractor, has begun to emerge; now, we're learning about the 12 other people killed in the shooting, which the Washington Post calls "the single worst loss of life in the District" since a 1982 plane crash. The shooting victims were ages 46 to 73, Mayor Vincent Gray says. All were civilians or military contractors, according to the New York Times, and DC police chief Cathy Lanier says at least one was a DC resident. Eight of the victims' names have been released, and the Post has details about their lives: Michael Arnold, 59: He served in the Navy for 29 years and retired as a captain last year. He worked for a government consulting firm and was at the Navy Yard working on a design for amphibious assault ships with a team. He has three sons with his wife of more than 30 years. Arthur Daniels, 51: He was at the Naval Yard for his job installing furniture in federal government buildings. His wife, with whom he has five kids and nine grandkids, calls him "a good father and a hard worker." Sylvia Frasier, 53: Her parents and siblings waited until 10pm last night for news of Frasier, a network-security administrator with the Naval Sea Systems Command. "He killed my sister," sobbed her youngest sister. Kathleen Gaarde, 62: She worked as a financial analyst at the Navy Yard and had just started planning her retirement with her husband of 38 years, with whom she had two children. "Now none of that matters," her husband says. "I already dearly miss her." John Roger Johnson, 73: A neighbor describes him as a civilian who worked for the Navy, and says he was a "smart man" and a "delightful neighbor." Frank Kohler, 50: No comment from his family, but a neighbor describes him as married with two daughters. Vishnu Pandit, 61: No comment from his family, but a neighbor says Pandit and his wife lived in the neighborhood for at least two decades and he was "a very nice man with an Irish setter." Kenneth Bernard Proctor, 46: He had worked for the federal government for 22 years; a civilian utilities foreman at the Navy Yard, he didn't work in the building where he was killed—but he often went there for breakfast. His ex-wife says they still talked every day, and had spoken that morning. They have two teen sons. Eight people were also injured, according to Gray, including three who were shot and five who suffered stress-related injuries; two of the injured were police officers.