Horrific: MIT police officer Sean Collier, 27, was found gurgling and covered in blood April 18, 2013 after prosecutors say he was shot in the head and neck by Dzhokhar Tsarnaev or his brother in a failed attempt to steal his gun . A Massachusetts Institute of Technology police officer who found a colleague in his squad car covered in blood three nights after the Boston Marathon bombing frantically repeated two words into his radio 'officer down, officer down.' A recording of that radio call was played to jurors hearing the trial of accused bomber Dzhokhar Tsarnaev, 21, on Wednesday, as prosecutors turned to the charge that the defendant and his older brother murdered MIT police officer Sean Collier on April 18, 2013, in an unsuccessful attempt to steal his gun. Meanwhile, the MIT police chief revealed his last words to the 27-year-old officer in a wrenching fifth day of testimony in the long awaited Boston bomber trial. Chief John DiFava on Wednesday recalled chatting with Collier about an hour before his death. 'I told him to be safe, and I left,' DiFava said, adding it was the last time he saw Collier alive. The officer who found Collier, Sergeant Clarence Henniger, said he was responding to a 911 call about possible shots fired on MIT's Cambridge, Massachusetts, campus, just outside Boston. Collier had visible bullet wounds to his temple, neck and hand and was so covered in blood that Henniger struggled to pull him from his vehicle. 'I could hear gargling coming from his mouth,' Henniger testified at U.S. District Court in Boston, where the jury hearing Tsarnaev's trial will also decide whether to sentence him to death if he is convicted of killing three people and injuring 264 with a pair of homemade bombs at the April 15, 2013, race. Jurors also saw surveillance footage of two figures approaching Collier's police cruiser on the MIT campus, spending about a minute at the car, and leaving. The only visible suggestions of commotion in the video are the car's brake lights, flashing on and off. Collier's gun belt and holster were covered in blood, evidence of Tsarnaev's clumsy but unsuccessful effort to steal his handgun, prosecutors have said. Tsarnaev's attorneys opened the trial by admitting he committed the crimes of which he is accused, but are seeking to spare him the death penalty by demonstrating he was following the lead of his older brother. Scroll down for video... Evidence: This forensics photograph released by the U.S. Attorney's office and presented during the trial of Dzhokhar Tsarnaev shows a bloodied service pistol sitting on seat of MIT Police Officer Sean Collier's cruiser . 'Be safe': MIT Police Chief John DiFava testified he told Collier to 'be safe' about an hour before he was shot dead April 18, 2013 . Culpable? Tsarnaev's attorney says Dzhohkar's brother Tamerlan was the mastermind of the bombing and the one who pulled the trigger to kill Collier. Prosecutors say both brothers were culpable in the officer's murder and that they killed the 27-year-old in order to steal his gun and arm Tsarnaev . In the act? Video taken from above the site of Collier's April 18 murder purports to show the Tsarnaev brothers (circled) rounding a corner toward Collier's cruiser, at left (grab via WCVB) Federal prosecutors contend Tsarnaev, who emigrated with his family from Chechnya a decade before the attack, was driven by an extremist view of Islam and a desire to take revenge on the United States for military campaigns in Muslim-dominated countries. Prosecutor William Weinreb said in his opening statement last week that it was unclear if Tsarnaev or his brother fired the weapon that killed Collier but argued that both brothers were equally culpable in the officer's death. A graduate student at MIT, Nathan Harman, who was bicycling on campus the night of the murder, testified on Wednesday that he saw Tsarnaev leaning into the driver's side door of Collier's car, but did not see the older sibling. 'I only saw the one person,' said Harman, referring to Dzhokhar. 'He sort of snapped up, stood up and turned around, and he looked startled…And then I just didn't think anything of it and rode off.' Aftermath: A damaged white jersey, worn by Boston Marathon bombing survivor Jessica Kensky during the blast, is displayed to the media in a conference room after the conclusion of the day's session . Explosion: The twisted, charred remnants of a homemade pressure cooker bomb that exploded during the Boston Marathon on April 15, 2013 . Pieces: The tattered remains of a white and black backpack carried by one the Boston bombers was collected following the terrorist attack . The court was also shown CCTV footage of Tsarnaev withdrawing $800 from an ATM using a credit card he had just stolen during a carjacking three days after the bombings . Harman said he assumed Tsarnaev was an MIT student. 'I remember thinking he had a big nose, but nothing beyond that really,' he said. Jurors also heard testimony on . Wednesday morning from an FBI agent who led the agency's efforts . to recover evidence from the debris-strewn blast sites. arrived hours after . explosions ripped through the race's crowded finished line in Boston April . 15, 2013. FBI Special Agent Sarah DeLair--who arrived hours afterexplosions ripped through the race's crowded finished line in Boston April15, 2013--held up a twisted piece of metal which had been found on Boylston Street and told the jury: 'It's part of a pressure cooker.' Other FBI agents on Tuesday described how some 3,000 pieces of evidence, including shrapnel and body parts, were retrieved from the blast sites near the marathon finish line, some on surrounding rooftops as high as four stories. Empty: The jury was shown a picture of the deserted courtyard behind MIT's Koch Institute. Officer Collier was gunned down as he pulled up next to the space. The city was put on lockdown in the wake of the attacks . Layout: A diagram presented by prosecutors depicts where Officer Collier's car was parked when he was shot in the head. CCTV footage shows two suspects fleeing the scene in the aftermath . Route: A wider drawing shows the area surrounding the scene of the deadly shooting. The two people scene running on camera headed towards the intersection between Main Street and Ames Street in the aftermath . FBI agent Sarah DeLair showed the court on Wednesday a piece of wire collected amid the wreckage on Boylston Street on April 15, 2013. 'It's the part of the bomb that would make it go off,' DeLair said . A blood-soaked zipper, nail and ball bearings found inside the pressure-cooker bomb which exploded at the marathon finish line. A survivor told the court earlier this week that the victims' injuries were difficult to heal and became infected because of the 'dirty' nature of the bomb . Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street in Boston in 2013 . Prosecutors also presented shredded pieces of a black and white backpack that they contend 21-year-old Dzhokhar Tsarnaev and his older brother, Tamerlan, used to carry their homemade bombs. The FBI agent also told the court that investigators found shrapnel along with ball bearings, a zipper, shards of metal, small nails and duct tape in the aftermath of the explosion. A bombing survivor told the court earlier this week that the victims' injuries were difficult to heal and became infected because of the 'dirty' nature of the bombs packed with small pieces of metal. DeLair showed  a piece of wire collected amid the wreckage on Boylston Street on April 15, 2013. The wire, she testified in U.S. District Court in Boston, was part of one of the bomb's detonators. 'It's the part of the bomb that would make it go off,' DeLair said. She took pictures of the . dead and coordinated efforts to recover 'everything from human . remains to bomb components to parts of backpacks,' she said on Wednesday. Prosecutors resumed questioning the FBI agent on Wednesday, the . fifth day of the high-profile trial. Tsarnaev, 21, is accused of killing three people and . injuring 264 with a pair of homemade bombs, as well as fatally . shooting a police officer three days later as he and his brother . tried to flee the city. His attorneys opened the trial by admitting he committed the . crimes of which he is accused, but are seeking to spare him the . death penalty by demonstrating he was following the lead of his . older brother Tamerlan, who died four days after the bombing . after a gun battle with police. Some 3,000 pieces of evidence, including shrapnel and body parts, were retrieved from the blast sites near the marathon finish line, some on surrounding rooftops as high as four stories . Federal prosecutors contend Tsarnaev, who emigrated with his . family from Chechnya, was driven by an extremist view of Islam . and a desire to strike back at the United States in revenge for . military campaigns in Muslim-dominated countries. On Tuesday, jurors were presented with photographs of the . blood-stained message that prosecutors say Tsarnaev wrote inside . the hull of a boat in which he was hiding in Watertown, outside . Boston, moments before his violent capture. The note accuses the United States government of killing . Muslims and says 'I can't stand to see such evil go unpunished'. It adds: 'I don't like killing innocent people it is forbidden in . Islam but due to said (...) it is allowed.' Words were missing . from the note due to bullet holes. Jurors were also presented with Tsarnaev's Twitter posts, . which ranged from jokes about girls, food, and homework, to . musings about Islam and a reference to the Sept. 11, 2001, . attacks on New York and Washington. After opening with three days of emotional and graphic testimony by witnesses including nine people injured in the attack, Tsarnaev's trial has moved into to a more technical phase as prosecutors show evidence about the bombs and communication between the two brothers. Despite the admission that opened the trial, Tsarnaev's not guilty plea stands, leaving it to the federal government to prove his guilt before the trial moves into a second phase, when the jury will determine whether to sentence him to death or life in prison without the possibility of parole. The Boston bombers are seen moving through the crowds at the 2013 marathon. Dzhokhar Tsarnaev's trial saw evidence on Wednesday including shreds of a backpack one brother was allegedly carrying .
Prosecutors say Dzhokhar Tsarnaev or his brother killed MIT officer Sean Collier, 27, days after the bombing in a failed attempt to steal his gun . Jurors were  shown CCTV footage of two suspects fleeing the scene . A photo of Collier's bloodied gun and holster was also presented in court . Sergeant Clarence Henniger testified to finding Collier shot and gurgling blood in his cruiser on April 18 in 2013 . Grad student Nathan Harman, 24, said that he saw Dzhokhar near the scene . In the fifth day of testimony, the jury was also shown remnants of the homemade bomb and a piece of a tattered black and white backpack .