The shooting death of beloved MIT campus cop Sean Collier was nothing more than a cold-blooded ambush by the Boston Marathon bombing suspects, authorities revealed yesterday — raising bedeviling questions about why the heavily hunted pair initiated their own undoing by, authorities allege, mercilessly executing a police officer.

Officials had previously said that Collier was responding to a “disturbance” at Main and Vassar streets about 10:30 p.m., but a spokesman for the Middlesex District Attorney’s Office yesterday corrected that statement, saying the officer “was not responding to anything.”

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

Collier’s shooting sparked the bloody, bizarre chain of events — authorities say they carjacked a Mercedes SUV in Cambridge, then tossed bombs and traded gunfire with cops during a furious shootout in Watertown — that ended in the death of suspect Tamerlan Tsarnaev, 26, and led to the daylong manhunt and eventual capture of his brother, Dzhokhar Tsarnaev, 19.

Massachusetts State Police Col. Timothy Alben said it baffled him to learn the suspects gunned down Collier with no provocation whatsoever.

“There’s no more logic to this than there is in placing a bomb behind an 8-year-old boy and women and blowing them to death,” Alben said. “It evades any kind of human decency whatsoever, and the same thing holds true with Officer Collier at MIT.”

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.

Yesterday saw an outpouring of sympathy for Collier, known on the Massachusetts Institute of Technology campus as a diligent patrolman who was about to become a Somerville cop.

“If he saw something was up, he would go straight to the source of it,” said Mike Okonkwo, an MIT researcher who often saw Collier while walking around campus. “He was friendly, but he was bold.”