Boston (CNN)It was 9:35 on a slow Thursday night in April 2013 and Massachusetts Institute of Technology Police Chief John DiFava was about to call it quits. On his way out, he saw one of his rookie swing-shift officers, Sean Collier, sitting in his cruiser. He stopped to say goodnight. 

"I chatted with him for a few minutes. I told him to be safe and I left," the chief told a crowded courtroom on Wednesday. He estimated the conversation lasted three, maybe four minutes. 

"Did you ever see Sean Collier alive again after that?" Assistant U.S. Attorney William Weinreb asked. 

"I did not." 

Less than an hour later, Collier lay bleeding in his patrol car after being ambushed and shot in the head. His car door was open, and his foot was lodged between the gas and brake pedals. 

DiFava and other officers, assisted by surveillance videos, 911 callers and a lone bicyclist who happened to be passing by, recounted Collier's last moments in the death penalty trial of admitted Boston Marathon bomber Dzhokhar Tsarnaev. 

The bicyclist, MIT mathematics Ph.D. candidate Nathan Harman, pointed to Tsarnaev in court and identified him as the man with "a big nose," who he saw leaning into Collier's squad car. He said Tsarnaev appeared to be alone. 

Tsarnaev, who was 19 at the time, does not dispute that he was present when Collier was killed on the evening of April 18, nor does he deny that he participated in the bombings three days earlier that killed three people and hurt more than 240 others. 
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Boston (CNN)