Beloved MIT campus cop Sean Collier “was not responding to anything” and appears simply to have been ambushed in his parked cruiser when he was shot to death late Thursday — sparking the bizarre and violent chain of events that led to the killing of one Boston Marathon bombing suspect and the capture of another, authorities said.

“He was just sitting in his car, sort of in a routine patrol location,” Middlesex District Attorney’s office spokeswoman Stephanie Chelf Guyotte said. “There’s no indication of whether they had a previous interaction. We do believe he was assassinated.”

Guyotte said previous reports that Collier was responding to a call for a disturbance were inaccurate.

Collier, 26, of Somerville, “was parked in an alleyway type of area” at Main and Vassar streets about 10:20 p.m. when Cambridge cops got a call reporting multiple gunshots, the DA’s office said.

Collier “did not return fire,” Guyotte said. “All indications are they approached the officer and they shot him.” His colleagues found him with multiple gunshot wounds, and he was pronounced dead at Massachusetts General Hospital.

Authorities say alleged terrorist brothers Tamelan and Dzhokhan Tsarnaev then carjacked a man at Third and Main streets, taking the wheel of his black Mercedes for half an hour before releasing him at a gas station on Memorial Drive at 12:19 a.m.

Police chased the car into Watertown, where they say the suspects tossed bombs out the car windows and exchanged more than 200 shots with cops. MBTA Transit cop Richard Donohue, 33, was injured. Tamelan Tsarnaev was killed.

Dzhokhan Tsarnaev fled under cover of night, leading authorities to cast a dragnet through a 20-square-block sector of Watertown between Mount Auburn and Arsenal streets. An unprecedented daylong manhunt — which shut down greater Boston — ended Friday evening, when authorities, acting on a tip from a resident, found Dzhokhan Tsarnaev cowering under a tarp in a boat on the resident’s property.