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. Related 
 
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 The Charges Against Dzhokhar Tsarnaev Explained in Plain English 
 
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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.)