The chief of the MIT police on Wednesday recalled the last time he saw Officer Sean Collier alive — telling him to "be safe" shortly before authorities say he was killed by the Boston Marathon bombers. 
 
 Chief John DiFava testified for the government at the trial of Dzhokhar Tsarnaev. He told the jury that he was leaving campus on the night of April 18, 2013, and spotted an MIT car on the street. 
 
 "I always make it a point to say hello," the chief testified. "I chatted with him for a few minutes. I told him to be safe, and I left." 
 
 He put the conversation at about 9:35 p.m., less than an hour before authorities have said Collier was shot to death in his patrol car. Three people were killed and more than 260 injured in the marathon bombings three days earlier. 
 
 Tsarnaev's lawyers have conceded that he took part in the bombings but say he was influenced by his older brother, Tamerlan, who authorities say was killed in the brothers' getaway attempt. 
 
 Sgt. Clarence Henniger of the MIT police described finding Collier in his cruiser, covered in blood, and telling him, "Hang in there, just hang in there." Jurors heard Henniger scream into his radio for medical help: "Get on it!" 
 
 A Cambridge police detective, Brendan O'Hearn, described performing chest compressions, trying in vain to save Collier's life. 
 
 Jurors on Wednesday also saw the tattered pieces of a backpack that Dzhokhar Tsarnaev may have used to carry one of the make-shift bombs. FBI Special Agent Sarah De Lair held up pieces of the black and white backpack for the jury, saying they were found near the Forum restaurant. 
 
 Dzhokhar Tsarnaev, now 21, faces the possibility of the death penalty if convicted. 
 
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 — Tom Winter, Andy Thibault and Erin McClam ||||| 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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– Jurors in the trial of Dzhokhar Tsarnaev heard about the murder of MIT police officer Sean Collier today, along with his chief's final exchange with him. As Chief John DiFava was leaving the campus, he spotted Collier's cruiser, reports NBC News. "I always make it a point to say hello," says the chief. "I chatted with him for a few minutes. I told him to be safe, and I left." That took place about 9:30pm on April 18, 2013. Less than an hour later, Collier was shot to death in his car, allegedly by the Tsarnaev brothers as they unsuccessfully tried to steal his gun. Video footage from the scene shows two men approaching Collier's car and his brake lights flickering, reports the Boston Globe. Then one of the two men extends an arm as a muzzle flashes and a bicyclist rides by. The latter appears to have been graduate student Nate Harman, who testified today that he was riding home and made eye contact with one of the two men by the car. Harman identified him as Dzhokhar Tsarnaev. (Previously, jurors got a close look at Tsarnaev's written messages inside the boat where he was hiding.)