Brian Snyder / Reuters Police officers take position during a search for the Boston Marathon bombing suspects in Watertown, Massachusetts April 19, 2013. Police on Friday killed one suspect in the Boston Marathon bombing during a shootout and mounted a house-to-house search for a second man in the suburb of Watertown after a bloody night of shooting and explosions in the city's streets. REUTERS/Brian Snyder (UNITED STATES - Tags: CRIME LAW) 
 
 Boston and its suburbs, universities and transit system were on total lockdown Friday as police hunted door to door for marathon bombing suspect Dzhokhar Tsarnaev -- on the loose after his accomplice brother was killed in a stunning chain of events that left one cop dead and another injured, officials said. 
 
 During a desperate effort to flee after their photos were released by the FBI, the brothers carjacked a Mercedes SUV and told the driver they were the men behind Monday's double-blast attack at the race and had just killed a campus security officer, a source told NBC News. The driver was released unhurt. 
 
 The older brother, Tamerlan Tsarnaev, was killed after a wild chase and firefight, but Dzhokhar was still at large in a densely populated area, and an associate was being sought. Adding to the nightmare, an explosive was found in Boston Friday morning and disabled, an official said. 
 
 The suspects' uncle, Ruslan Tsarni, called them "losers" and urged Dzhokhar, a naturalized American citizen of Chechen origin, to turn himself in. 
 
 "We're ashamed," he thundered outside his Maryland home. 
 
 Three dozen FBI agents were surrounding the Cambridge, Mass., home where the brothers grew up after moving to the U.S. a decade ago, seeking asylum. They feared it could be booby-trapped and planned to conduct a controlled explosion before a full search, state police said. 
 
 NBC's Lester Holt reports from Watertown, Mass., where Boston and state police, SWAT teams and a helicopter are engaged in a tense search for a suspect in the Boston Marathon bombing who is still at large. 
 
 Across the area, as police cars screamed down streets and helicopters hovered overhead, authorities urged the public to stay inside and keep their doors locked to anyone but law-enforcement officers. 
 
 "There is a massive manhunt under way," Massachusetts Gov. Deval Patrick said. "We are asking people to shelter in place." 
 
 Subways and buses were shut down, and Amtrak service to Boston was cut. Harvard University, Boston University, the Massachusetts Institute of Technology and Emerson University were closed. The University of Massachusetts' Dartmouth campus was being evacuated because someone wanted in the case is registered at the school. 
 
 The lockdown initially affected more than 300,000 people in Cambridge, Watertown, Newton, Brighton, Allston and Belmont, but by 8 a.m., the entire city of Boston was paralyzed, officials said. 
 
 Watertown, where the second suspect was last seen, was the epicenter of a sprawling search. By 12:30 p.m., police estimated, 60 percent of the homes had been searched. Frightened residents were trapped inside as convoys of heavily armed officers and troops arrived by the hour. 
 
 The overnight violence began at MIT about five hours after the FBI released surveillance photos of two "extremely dangerous" men suspected of planting two bombs near the finish line of Monday's Boston Marathon, killing three and wounding 176. 
 
 Read more: Who are the brothers accused of the Boston Marathon bombing? 
 
 Tips about the identity of the suspects were still pouring in when the Tsarnaev brothers fatally shot MIT patrol officer Sean Collier, 26, in his vehicle at 10:20 p.m., law enforcement officials said. 
 
 Uncredited / AP FBI photos of the Boston Marathon bombing suspects, brothers from the Chechen Republic. The man on the left is dead. The man on the right, Dzhokhar Tsarnaev, is still at large. 
 
 The brothers then carjacked a Mercedes SUV, holding the driver captive for a half-hour while they tried to use his cash card to get money from three ATM's, a source said. At the first, they put in the wrong number; at the second, they took out $800 and at the third, they were told they had exceeded the withdrawal limit, the source said. 
 
 The man was released unharmed at a gas station in Cambridge, sources said. As they sped toward Watertown, a police chase ensued and the suspects tossed explosive devices out the window, officials said. 
 
 There was a long exchange of gunfire, according to Andrew Kitzenberg of Watertown, who took photos of the clash from his window and spread them on social media. 
 
 “They were also utilizing bombs, which sounded and looked like grenades, while engaging in the gunfight,” he told NBC News in an interview. “They also had what looked like a pressure-cooker bomb. 
 
 “I saw them light this bomb. They threw it towards the officers,” he said. “There was smoke that covered our entire street.” 
 
 A transit officer, identified as Richard H. Donahue, 33, was seriously injured during the pursuit. Authorities said he was in surgery at Mount Auburn Hospital. 
 
 Kitzenberg said he saw the firefight end when Tamerlan Tsarnaev ran toward the officers and ultimately fell to the ground. 
 
 Tamerlan -- the man in the black hat from FBI photos released six hours earlier -- had an improvised explosive device strapped to his chest, law enforcement officials said. 
 
 Dzhokhar -- who was wearing a white hat in the surveillance photos from the marathon -- drove the SUV through a line of police officers at the end of the street, Kitzenberg said. 
 
 Police said Dzhokhar has a Massachusetts driver's license and lives in Cambridge. He was described as light-skinned and with brown, curly hair, and wearing a gray hoodie. The FBI released more photos of him, including a surveillance camera photo from a 7-Eleven. 
 
 His father, in Russia, told the Associated Press he was "a true angel" and described him as a medical student who was expected to visit for the holidays. 
 
 Authorities painted a starkly different picture. 
 
 "We believe this man to be a terrorist," Boston Police Commissioner Ed Davis said. "We believe this to be a man who's come here to kill people." 
 
 Armored humvees and busloads of law-enforcement could be seen rolling into Watertown in the hours after the gunfight. 
 
 "We've got every asset we could possibly muster on the ground right now," Patrick said. 
 
 Later Friday morning, attorney General Eric Holder and FBI Director Robert Mueller headed to the White House to brief President Obama on the developments. 
 
 Secretary of State John Kerry, who is from Massachusetts, praised law enforcement for their work during "a pretty direct confrontation with evil." 
 
 "In the past few days we have seen the best and we've seen the worst of human behavior, and it's the best that all of us really want to focus on," he said. 
 
 Senate Intelligence Committee chairwoman Dianne Feinstein told NBC News it's clear the marathon bombing was "a terrorist attack." 
 
 "It's also clear, you know, that you know you don't have to be a card-carrying member of any terrorist group to commit a terrorist attack," she said. 
 
 NBC News' Jonathan Dienst and Kasie Hunt contributed to this story 
 
 Slideshow: Search for suspects in Boston Marathon bombings / A tense night of police activity that left a university officer dead on campus just days after the Boston Marathon bombings and amid a hunt for two suspects caused officers to converge on a neighborhood outside Boston, where residents heard gunfire and explosions. Launch slideshow 
 
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 Who are the brothers accused of the Boston Marathon bombing? 
 
 An empty metropolis: Photos show deserted streets of Boston 
 
 What we know: Timeline of terror hunt 
 
 ‘Dedicated officer’ gunned down by Boston Marathon suspects at MIT 
 
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 This story was originally published on ||||| Boston manhunt aftermath: Slain MIT officer remembered while transit officer and friend fights for life Massachusetts Institute of Technology officer Sean Collier, 26, was killed at the hands of the Boston Marathon bombing suspects, the Tsarnaev brothers, police said. Meanwhile, his fellow police academy grad and friend, transit cop Richard Donohue Jr., remains in serious condition in the hospital. 
 
 A photo released Sunday by the Massachusetts Bay Transportation Authority shows Richard Donohue Jr. (l.) and MIT officer Sean Collier (r.) at their graduation from the Municipal Police Officers’ Academy in 2010. 
 
 They were brothers in blue, each with promising futures. 
 
 A 2010 police academy graduation photo showing Sean Collier with his arm around Richard Donohue Jr. depicts a hopeful bond — and no hint that the friends would forever be linked by tragedy. 
 
 Collier, a 26-year-old Massachusetts Institute of Technology police officer, was fatally shot Thursday at the hands of Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev, authorities said. 
 
 JIM BOURG/REUTERS 
 
 Donohue, an officer with the Massachusetts Bay Transportation Authority, was injured during an ensuing shootout between the Tsarnaevs and cops. 
 
 PHOTOS: INSIDE THE MANHUNT FOR BOSTON MARATHON BOMBING SUSPECTS 
 
 Collier's body was brought to a funeral home Saturday. The actual funeral is still being planned. 
 
 DOMINICK REUTER/REUTERS 
 
 Donohue, 33, remains in serious condition at Mount Auburn Hospital in Cambridge, Mass., his family said in a release. 
 
 “Please continue to pray for Richard, a wonderful father, husband, son and brother, and a dedicated police officer who loves his work and helping people,” the statement read. 
 
 As Donohue continued to fight for his life, several thousand people gathered for a vigil in Collier’s hometown of Wilmington, Mass., to remember the young cop who was expecting to join the Somerville Police Department in June. 
 
 AFP PHOTO / Massachusetts Institute of Technology 
 
 RELATED: SLAIN MIT COP VOLUNTEERED AT TAMERLAN TSARNAEV'S BOXING GYM 
 
 “His dream was coming true,” said Somerville Deputy Police Chief Michael Cabral, according to the Los Angeles Times. “As you know, that dream was crushed.” 
 
 Collier’s mother didn’t attend the vigil, because she was still emotionally devastated. 
 
 MBTA/Reuters 
 
 Other friends and family members spoke of Collier’s dedication to the job and the community. He recently asked for permission to join the board of a local homeless shelter, MIT Police Chief John DiFava told those at the vigil. 
 
 Collier never had a fighting chance against his attackers. 
 
 RELATED: BOSTON MARATHON BOMBINGS MANHUNT: A TIMELINE 
 
 DOMINICK REUTER/REUTERS 
 
 Authorities say the Tsarnaev brothers ambushed him while he responded to a disturbance on the MIT campus late Thursday, three days after the deadly carnage at the Boston Marathon. 
 
 He was hit possibly five or six times. His gun was still in the holster. 
 
 The Tsarnaev brothers then carjacked a man and led police in a pursuit with guns blazing, officials said. Tamerlan Tsarnaev died in the gun battle, while Donohue, an MBTA Transit cop of three years, was shot once. 
 
 “These are the bravest men and women I know,” Collier’s stepfather, Joe Rogers, said at Saturday’s vigil, reminding those in attendance that officers pay the biggest sacrifice. 
 
 “Every day they go out to keep us safe from the people who do evil in the world," Rogers said. "I think we need to applaud them because one night it could be them walking down a dark alley.” 
 
 eortiz@nydailynews.com 
 
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– Authorities say one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is under way for the other, who has been identified as 19-year-old Dzhokhar A. Tsarnaev. Boston is effectively shut down in the meantime: Residents of Boston, Watertown, Newton, Waltham, Belmont, Cambridge, and the Allston-Brighton neighborhoods of Boston were told to remain in their homes; mass transit has been halted; businesses were asked not to open; and the Boston Globe reports that at least 16 area colleges have canceled classes. What we know of what happened overnight, per the AP and the Washington Post: The Middlesex district attorney says the two men are suspected of killing an MIT police officer at the college last night after the officer responded to a disturbance, then stealing a car at gunpoint and later releasing its driver unharmed. (All just hours after police released photos of the bombing suspects.) Authorities say the suspects threw explosives from the car as police followed it into Watertown; witnesses reported hearing multiple gunshots and explosions at about 1am. A transit police officer was critically wounded. The suspects and police exchanged gunfire and one of the suspects was critically injured and later died at a hospital while the other escaped. Doctors at Boston's Beth Israel Deaconess Medical Center say they treated a man with a possible blast injury and multiple gunshot wounds. They wouldn't confirm that the patient, who came in with police, was the suspect in the black hat from marathon surveillance footage. Police say the suspect on loose is a "terrorist" who "came here to kill people." Governor Deval Patrick held a press conference with police this morning extending the shelter-in-place warning to all of Boston. "There is a massive manhunt underway," he assured reporters. "We've got every asset that we can possibly muster on the ground right now." Authorities have identified the MIT officer who was killed as 26-year-old Sean Collier of Somerville, according to the Boston Globe. The MBTA police officer who was injured has been identified as Richard H. Donohue.