article_snippet: Massachusetts Institute of Technology Police Chief John DiFava and other officers, assisted by surveillance videos, 911 callers and a lone bicyclist who happened to be passing by, recounted Sean Collier's last moments in the death penalty trial of admitted Boston Marathon bomber Dzhokhar Tsarnaev.
article_snippet: Boston Marathon bomber Dzhokhar Tsarnaev said admitted Boston Marathon bomber Dzhokhar Tsarnaev appeared to be alone.
article_snippet: Tsarnaev, who was 19 at the time, does not dispute that admitted Boston Marathon bomber Dzhokhar Tsarnaev was present when Sean Collier was killed on the evening of April 18, nor does admitted Boston Marathon bomber Dzhokhar Tsarnaev deny that admitted Boston Marathon bomber Dzhokhar Tsarnaev participated in the bombings three days earlier that killed three people and hurt more than 240 others.
article_snippet: Prosecutors say admitted Boston Marathon bomber Dzhokhar Tsarnaev and admitted Boston Marathon bomber Dzhokhar Tsarnaev older brother, Tamerlan Tsarnaev, killed Sean Collier because they wanted Sean Collier gun.
article_snippet: Sean Collier became the fourth victim of the Tsarnaev brothers.
highlights_snippet: Sean Collier killed three days after Boston bombings .
  