- The violence began late Thursday, April 18, with the robbery of a convenience store in Cambridge, according to Massachusetts State Police spokesman Col. Timothy Alben. Police initially indicated that it was a 7-Eleven convenience store, but later reports suggested otherwise.
- Soon after, a Massachusetts Institute of Technology police officer — identified as Patrol Officer Sean A. Collier — was fatally shot, the Suffolk County District Attorney’s Office said in a statement.
- Collier was found shot to death in his vehicle in the vicinity of Vassar and Main streets on the MIT campus, according to an MIT statement.
- Authorities say the two suspects then hijacked a car at gunpoint in the area of Third Street in Cambridge, according to a statement from Middlesex District Attorney’s Office.
- The driver of the carjacked vehicle escaped a half-hour later at a gas station on Memorial Drive in Cambridge, a law enforcement source told CNN.
- Police, who were tracking the vehicle using its built-in GPS system, picked up the chase in Watertown, about four miles west of the reported area of the carjacking. The pursuit went into a residential neighborhood, with the suspects throwing explosives at the police, according to the law enforcement source. The exchange of gunfire occurred in the area of Dexter and Laurel streets, according to the Middlesex County District Attorney’s Office.
- During the firefight, a suspect — later identified as Tamerlan Tsarnaev, 26 — got out of the car. Police shot him, and his brother ran over him as he drove away, according to a law enforcement source. A source briefed on the investigation says Tamerlan Tsarnaev was wearing explosives and an explosive trigger. He died later at Beth Israel Hospital.
- A Boston transit police officer, Richard H. Donohue, Jr., was injured and transported to a hospital, authorities said.
Suspect 2 is at large, possibly in Watertown, where dozens of officers from local, state and federal law enforcement agencies have fanned out to track him down. The suspect has been identified as Dzhokar Tsarnaev, 19, several sources told CNN. Police are going door-to-door to look for him.
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