That officer, later identified as 26-year-old Sean Collier, was responding to a report of a disturbance Thursday night when he was shot multiple times. Law enforcement officers told ABC News that the marathon bombing suspects, 26-year-old Tamerlan Tsarnaev and 19-year-old Dzhokhar Tsarnaev, killed Collier in cold blood while the officer was in his car.
The shooting took place at about 10:30 p.m. ET outside an MIT building, according to a state police spokesperson. Collier was rushed to an area hospital, where he was pronounced dead.
Shortly after, the Tsarnaev brothers carjacked a Mercedes-Benz SUV in Cambridge. They apparently identified themselves as the marathon bombing suspects to the carjacking victim. That victim alerted authorities and a chase ensued, making its way into Watertown, Mass.
During that chase, a gun battle erupted between the suspects and police - and Watertown residents found themselves caught in the middle of the violence.
Dozens of rounds were fired as police cornered the suspects. Witnesses say the suspects were also throwing explosives at them.
That's when authorities say the older suspect, Tamerlan Tsarnaev, was shot and killed. Tamerlan was rushed to a local hospital where doctors say he died of multiple gunshot wounds. Authorities also said he had wounds caused by explosives. This is raising the possibility that he may have had a suicide vest on.
Since that shooting, the noose has tightened around the community of Watertown, where authorities think the younger brother still may be located.
A large area of town has been ordered to "shelter in place," as residents are being told to stay in their homes. Police are concerned that Dzhokhar Tsarnaev may not only be armed with an assault rifle but may also have explosives. That is one reason why they are being so cautious.
The Associated Press and ABC News contributed to this report.