Night of terror
The violence erupted when a college police officer was shot and killed late Thursday. The events that followed sent sirens howling through the night, and emergency lights shattering the darkness, as police chased after two suspects who tossed explosives at them.
Still on edge from Monday's deadly bombings, local, state and federal law officers responded to reports of a shooting on the campus of the Massachusetts Institute of Technology in Cambridge directly across from Boston on the Charles River.
At the time, they did not know of possible connections to the bomb attacks.
News media from across the country -- in Boston to cover the investigation into the marathon terror -- flocked to MIT's campus.
Two men had shot and killed the university policeman, while he sat in his car, the district attorney's office later said. State police and the FBI found the officer there with multiple gunshot wounds.
He was transported to Massachusetts General Hospital, where he was pronounced dead.
The same two suspects then hijacked a car at gunpoint in Cambridge and initially held the vehicle's driver in the car with them, the district attorney said. They released him half an hour later at a gas station unharmed.
Police encountered the stolen vehicle and gave chase. Suspects threw explosives out the window at the officers in a futile attempt to elude capture. "The suspects and police also exchanged gunfire," the statement read.
A transit police officer was "seriously injured" and taken to hospital.
Showdown in Watertown
Police cornered the vehicle in the suburb of Watertown and set up a perimeter, as reinforcements and emergency vehicles poured into the area.
Officers quickly locked down the streets of the Watertown neighborhood after isolating the vehicle.
Police carrying assault rifles ran down the streets, according to CNN affiliate WCVB, which broadcast images from the area.
One of the suspects -- the man who could be suspect number 1 -- had been wounded during the pursuit. He was taken to Beth Israel hospital, where Doctors pronounced him dead after unsuccessful attempts to reanimate him, a hospital spokesman from said.
He had bullet wounds and injuries from an explosion.
CNN photographer Gabe Ramirez arrived in Watertown as the chase ended.
"Police were in a standoff with the vehicle just down the hill," Ramirez said. They ordered one suspect out and commanded him to strip down completely naked before putting him in a patrol car, which did not leave the scene.
The man was later released and is not a suspect in the case.