(CNN) -- Two men suspected in the Boston Marathon bombings that killed three people may have terrorized the area a second time overnight. The latest developments:
--The man known as "Suspect 1" in the bombings was shot and killed by officers in an exchange of gunfire, Gov. Deval Patrick said.
--Several sources tell CNN that the dead suspect has been identified as Tamerlan Tsarnaev, age 26.
-- "Suspect 2" is at large in the Boston suburb of Watertown, where dozens of officers from local, state and federal law enforcement agencies have fanned out to track him down.
-- The suspect at large is Dzhokhar Tsarnaev, age 19, several sources told CNN.
-- Police are going door-to-door to look for him.
--The suspects were brothers, a terrorism expert briefed on the investigation said.
--The suspects are originally from the Russian Caucasus and had moved to Kazakhstan at a young age before coming to the United States several years ago, according to a source briefed on the investigation.
-- The violence began late Thursday night when a Massachusetts Institute of Technology police officer was shot and killed when he responded to a disturbance on campus.
--Another policeman, transit officer Richard H. Donohue Jr., was also shot and wounded overnight. The 33-year-old is a three-year veteran of the force, a transit police spokesman said Friday.
-- Within an hour, police were called to respond to a carjacking, which led them on a chase of the two suspects through Watertown.
-- The fleeing men allegedly used explosives against police as they were being chased.
-- Police have warned Watertown residents to lock their homes and stay away from windows and doors.
CNN's Joe Sterling contributed to this report