article_snippet.sent: Massachusetts Institute of Technology Police Officer Sean Collier would become the fourth victim of the Tsarnaev brothers.
article_snippet.sent: U.S. Prosecutors said the Tsarnaev brothers killed Massachusetts Institute of Technology Police Officer Sean Collier because the Tsarnaev brothers wanted Massachusetts Institute of Technology Police Officer Sean Collier gun.
article_snippet.sent: But the Tsarnaev brothers efforts to take it were thwarted by a safety holster.
article_snippet.sent: Tamerlan, 26, was killed in a chase and gunbattle with police that began with reports of an "officer down" at MIT.