This undated photo provided by the Houston Police Department shows Joseph James Pappas. Authorities on Wednesday, Aug. 1, 2018, identified a man who they believe gunned down one of former President George... (Associated Press) 
 
 This undated photo provided by the Houston Police Department shows Joseph James Pappas. Authorities on Wednesday, Aug. 1, 2018, identified a man who they believe gunned down one of former President George H.W. Bush's doctors last month as Pappas, the son of a woman who died while the doctor was operating... (Associated Press) 
 
 This undated photo provided by the Houston Police Department shows Joseph James Pappas. Authorities on Wednesday, Aug. 1, 2018, identified a man who they believe gunned down one of former President George H.W. Bush's doctors last month as Pappas, the son of a woman who died while the doctor was operating... (Associated Press) This undated photo provided by the Houston Police Department shows Joseph James Pappas. Authorities on Wednesday, Aug. 1, 2018, identified a man who they believe gunned down one of former President George... (Associated Press) 
 
 HOUSTON (AP) — A man suspected of killing one of former President George H.W. Bush's doctors may have been seeking revenge for his mother, who died on the doctor's operating table more than 20 years ago, authorities said Wednesday. 
 
 Joseph James Pappas, 62, should be considered armed, dangerous and possibly suicidal, Houston police Chief Art Acevedo said at a news conference Wednesday afternoon. 
 
 "There was a lot of planning that went into this. There was a lot of planning and, sadly, some skill," Acevedo said of the July 20 attack on Dr. Mark Hausknecht. Hausknecht was gunned down while riding his bike to work at Houston Methodist Hospital, which is part of the busy Texas Medical Center. Authorities say the shooter rode past Hausknecht before turning around and firing. 
 
 A tip on Tuesday led police to suspect Pappas, the chief said, though he didn't elaborate as to the nature of the tip. 
 
 Acevedo said Pappas hadn't been seen in 36 to 48 hours. He said the last anyone had heard from Pappas was in a Tuesday morning text message in which Pappas wrote that he was going to kill himself. 
 
 Acevedo said police searched his home at 4 a.m. Wednesday and found evidence that ties Pappas to the killing, but he declined to elaborate as to the nature of the evidence. 
 
 The chief said Pappas is white and very fit, and that he's likely getting around on his 10-speed bicycle, which he rides "extensively and almost exclusively." 
 
 Friends and family have described Hausknecht as a humble and generous man who was adored by his patients, volunteered in his community and cared about the environment. 
 
 Hausknecht was also an avid cyclist and rode his bike to work each morning, as he lived less than 2 miles (3.2 kilometers) from his office. 
 
 In 2000, Hausknecht treated Bush for an irregular heartbeat after the ex-president complained about lightheadedness. 
 
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 Follow Juan A. Lozano on Twitter: https://twitter.com/juanlozano70 ||||| Man with a grudge over mother’s death sought in slaying of Houston doctor 
 
 Houston Police Department chief Art Acevedo releases a photo of Joseph James Pappas, the suspect in the murder of Dr. Mark Hausknecht, during a press conference at HPD headquarters Wednesday, Aug. 1, 2018, in Houston. less Houston Police Department chief Art Acevedo releases a photo of Joseph James Pappas, the suspect in the murder of Dr. Mark Hausknecht, during a press conference at HPD headquarters Wednesday, Aug. 1, 2018, in ... more Photo: Mark Mulligan, Houston Chronicle Photo: Mark Mulligan, Houston Chronicle Image 1 of / 30 Caption Close Man with a grudge over mother’s death sought in slaying of Houston doctor 1 / 30 Back to Gallery 
 
 After apparently nursing a grudge for 20 years, former constable deputy Joseph James Pappas allegedly got on his 10-speed Schwinn bicycle, rode to the Texas Medical Center and gunned down Houston cardiologist Mark Hausknecht in broad daylight, authorities said. 
 
 Police late Wednesday were searching for the accused killer whose mother died on the operating table under Hausknecht's care two decades ago. The 62-year-old may be suicidal and should be considered armed, according to Houston Police Chief Art Acevedo. 
 
 "I don't know when he decided to do this, but I'm very familiar with how this murder was committed and there was a lot of planning that went into this," Acevedo said during an afternoon news conference to announce charges against Pappas. "There was a lot of planning and sadly some skill." 
 
 Five days after Hausknecht's slaying, a man using the same phone number as Pappas tried to sell a stash of weapons, ammunition and tactical vests online. When police searched his Westbury home early Wednesday, he already had given away many of his belongings, according to sources close to the investigation, but he left behind a gun and several holsters. 
 
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 For nearly two weeks, the case baffled police, who pored through surveillance footage as they struggled to solve the brazen July 20 killing. That morning during rush hour, Hausknecht was biking near the intersection of Main and Holcombe when the another man on a bike passed him from behind. Two blocks later, the man turned around on his 10-speed Schwinn bike and fired at least twice. The doctor — a famed physician who once treated former President George H.W. Bush — went down immediately. The gunman rode off in a northbound direction. 
 
 Afterward, Crime Stoppers offered a $5,000 reward for information, but authorities remained tightlipped about their investigation, save for a slow trickle of surveillance video and still photographs from the moments before and after the killing. 
 
 On Monday, one of those videos broke the case. A tipster alerted police to a possible connection to Pappas, whose mother was one of Hausknecht's patients. 
 
 "It appears that this may have been a 20-year-old grudge that this man held," Acevedo told reporters. 
 
 Police kept investigating and eventually learned that Pappas had texted someone he knew indicating he wanted to kill himself, and that he hadn't been heard from in more than a day. Late Tuesday, officers showed up at his home for a welfare check, but Pappas was not there. 
 
 They returned around 4 a.m. Wednesday with a pair of warrants — one to search the house and another to arrest Pappas for murder. 
 
 They did not find their suspect, but they did locate evidence tying him to the killing. 
 
 "We believe that this man is absolutely the killer," Acevedo said, declining to offer more details. As of late Wednesday, Pappas still had not been found. 
 
 "We need to find this guy," the chief said. "This man is dangerous, this man is capable, this man has some skills." 
 
 Until about five years ago, Pappas worked in Harris County law enforcement for roughly three decades, according to state records. From 1997 to 2013, he served as a reserve officer with Harris County Constable Precinct 2, after a number of years on the force there and at Precinct 7. 
 
 At HoustonChronicle.com: Slain Houston doctor remembered at memorial service 
 
 Pappas also had an active real estate license with a business address on Westheimer, and a phone number matching several online weapon sales listings. A call to that number Wednesday went straight to voicemail. 
 
 The items for sale, posted July 25, include two tactical vests, a Smith & Wesson .38-caliber revolver, a $4,500 semi-automatic FN rifle, two sets of ballistic door panels for a Crown Victoria and a box of ammunition. 
 
 An ad on another site in January lists the seller's name as "Joe," notes a southwest Houston location, and offers a variety of ammunition reloading equipment. 
 
 It was unclear whether the guns for sale included the weapon used in the doctor's fatal shooting. Asked for comment, police said they would investigate. 
 
 "We are looking at all aspects of the suspect's life and activities," Acevedo said in a text late Wednesday. 
 
 "It is highly probable he was targeted," Acevedo said in a text message Thursday morning. 
 
 The suspected shooter's neighbors had little to say about him. One woman said she knew he had grown up there, but that they only made neighborly chit-chat and and that he seemed normal. 
 
 A former acquaintance, Yvonne Shaw, said she had stayed with Pappas at his apartment in Houston for about two weeks more than 15 years ago. He told her he worked as an undercover police officer. He seemed caring, she said, but did not like to talk about his job or his past. 
 
 "I couldn't figure anything out, and I didn't want to ask him too much because I hardly knew him," Shaw said, later adding, "I thought he was a little secretive about that stuff." 
 
 The two lost contact when she left Texas, and she had not heard from him since. 
 
 "To me, he was a very nice person," Shaw said. "He cared about people. He didn't seem like someone who would hurt anybody." 
 
 He never talked about his family or his relationship with his mother, she said. However, she said he frequently rode a 10-speed bicycle. 
 
 "He always had it inside so no one would steal it," Shaw said. 
 
 TRIBUTES: Family, friends fondly recall doctor shot while on bike 
 
 Grudge-motivated violence against health-care providers is rare but not unheard of. A 2012 study in Annals of Emergency Medicine identified 154 hospital-related shootings between 2000 and 2011, of which more than 27 percent involved a "determined shooter" harboring a grudge. The victim much more often was a hospital employee than a doctor. 
 
 "Sometimes the hardest part of our job isn't diagnosing or treating the disease, but communicating the gravity of the patient's circumstances," said Dr. Douglas Curran, president of the Texas Medical Association. "It's a struggle. Loved ones can have a hard time comprehending that we can't fix everybody." 
 
 It was not clear what may have prompted the accused killer to act so many years after his mother's death. 
 
 "Never underestimate how deeply people care about and feel the loss of a loved one," said Dr. Edward Poa, a forensic psychiatrist at The Menninger Clinic. "If they blame someone for that loss, they can really want retribution, an eye-for-an-eye sort of thing. They can see their act as righting a wrong." 
 
 The homicide stunned members of the Texas Medical Center Community, including at Houston Methodist, where Hausknecht was an interventional cardiologist. Such specialists treat cardiovascular disease and congenital and structural heart conditions through catheter-based procedures, such as angioplasty and stenting. 
 
 "The Houston Methodist family is still in shock about the senseless killing of one of our longtime physicians, Dr. Mark Hausknecht," said Marc Boom, president and CEO of Houston Methodist. "We are relieved that the Houston Police Department has identified the suspect and are confident he will be apprehended soon." 
 
 Hausknecht, 65, earned his degree from Baylor College of Medicine in 1980, and started practicing in Houston seven years later. He biked to work every day, and took exceptional care of himself, according to those who knew him. 
 
 In 2000, Hausknecht appeared at a news conference at Methodist with Bush after the 41st president was treated for an irregular heartbeat. 
 
 A spokesman for the former president offered prayers and condolence to the doctor's family, colleagues and supporters. 
 
 "Mark was a fantastic cardiologist and a good man," Bush said in a statement. "I will always be grateful for his exceptional, compassionate care. His family is in our prayers." 
 
 The family held a funeral Saturday at First Presbyterian Church. In addition to his wife, Hausknecht is survived by two sons, his mother and three siblings. 
 
 Julian Gill, Fernando Alfonso, Stephen Tucker Paulsen, Victoria Cheyne and Nancy Sarnoff all contributed to this report. ||||| (CNN) A man whose mother died during surgery 20 years ago is the suspect in the fatal shooting of a prominent Houston cardiologist who performed the operation, police said Wednesday. 
 
 Chief Art Acevedo said the mother of Joseph James Pappas, 62, was a patient of Dr. Mark Hausknecht , who was fatally shot as he rode his bicycle to work at Houston Methodist Hospital on July 20. Police earlier said Pappas was born in 1953. 
 
 Houston Police released a photo of Joseph James Pappas, who is a suspect in the death of Dr. Mark Hausknecht. 
 
 "So, it appears that this may be a 20-year-old grudge that this man held," Acevedo told reporters. 
 
 CNN called a phone number listed for Pappas' real estate company on Wednesday, but the call went to voice mail. 
 
 That phone number also is linked to recent listings on a firearms auction website for several guns, ammunition, tactical vests and ballistic plates for car doors. The posts, from a user in Houston, apparently went online days after Hausknecht was shot. 
 
 It is unclear whether any of the firearms listed online were used in the shooting. 
 
 A spokesman for Houston police would not comment on the ads. 
 
 Suspect likely owned several guns, chief says 
 
 Acevedo said a tip came in Tuesday after surveillance video released a day before showed the suspect, whom someone said they recognized. Police got a search warrant and went to Pappas' home, but he was not there. 
 
 Acevedo said Pappas had made phone calls indicating he was considering suicide. The suspect has not been seen in almost two days. 
 
 "This man is dangerous. This man is capable. This man has some skills (with firearms)," he said. 
 
 There was evidence at the home linking Pappas to the killing, the chief said. A lot of planning went into the killing, according to Acevedo. 
 
 The suspect appeared to own multiple guns, he said. 
 
 Pappas worked for 30 years as a constable, according to Texas Commission on Law Enforcement records. 
 
 He worked for both Harris County Constable Precincts 2 and 7 as a peace officer and a reserve officer from May 1983 to July 2013. 
 
 Pappas took law enforcement classes through August 2017. 
 
 Video captured images of suspect 
 
 The renowned cardiologist, who treated former President George H.W. Bush, and the suspect were seen shortly before the shooting riding their bicycles on South Main Street, police said. 
 
 This surveillance image shows a suspect in the shooting of Dr. Mark Hausknecht, police say. 
 
 Surveillance footage showed the suspect following Hausknecht as both pedaled north, police said. 
 
 Hausknecht's wife, Georgia Hsieh, said police told her that her husband was riding through a construction area when the gunman rode up in front of him and fired three shots while facing him. 
 
 The cardiologist was found feet away from the construction site, where about 500 workers were on duty at the time, a construction manager at the site said. But no one heard anything because of the equipment. 
 
 Hausknecht, circled in green, was followed by a cyclist, in red, shortly before the shooting, police say. 
 
 A close-up surveillance image showed the suspect wearing a blue, short-sleeve polo shirt, a khaki or white cap, khaki shorts and an olive-green backpack, police said. 
 
 The suspect, circled in red, nears Hausknecht on the road. 
 
 Other surveillance images showed the suspected gunman just behind Hausknecht as both headed north on Main across West Holcombe Boulevard, police said. 
 
 Police highlighted Hausknecht with a green circle and the suspect with a red circle on images posted on Twitter. The fatal shooting happened moments after that crossing, one block away, police said.
– A man suspected of killing one of former President George HW Bush's doctors may have been seeking revenge for his mother, who died on the doctor's operating table more than 20 years ago, authorities said Wednesday. Joseph James Pappas, 62, should be considered armed, dangerous, and possibly suicidal, Houston police Chief Art Acevedo said at a news conference Wednesday afternoon. "There was a lot of planning that went into this," Acevedo said of the July 20 attack on Dr. Mark Hausknecht, adding that it appears a "20-year-old grudge" led to the attack, per CNN. Hausknecht was gunned down while riding his bike to work at Houston Methodist Hospital, which is part of the busy Texas Medical Center. Authorities say the shooter rode past Hausknecht before turning around and firing. A tip on Tuesday led police to suspect Pappas, the chief said, though he didn't elaborate as to the nature of the tip. Acevedo said Pappas hadn't been seen in 36 to 48 hours, the AP reports. He said the last anyone had heard from Pappas was in a Tuesday morning text message in which Pappas wrote that he was going to kill himself. Acevedo said police searched his home at 4am Wednesday and found evidence that ties Pappas to the killing, but he declined to elaborate as to the nature of the evidence. The chief said Pappas, who was a peace officer in Harris County for three decades per the Houston Chronicle, is white and very fit, and that he's likely getting around on his 10-speed bicycle, which he rides "extensively and almost exclusively." Friends and family have described Hausknecht as a humble and generous man who was adored by his patients, volunteered in his community and cared about the environment. He was also an avid cyclist and rode his bike to work each morning, as he lived less than 2 miles from his office.