"The suspects and police also exchanged gunfire in the area of Dexter and Laurel streets. During this pursuit, an (Massachusetts Bay Transportation Authority) Police officer was seriously injured and transported to the hospital," Boston police said in a release.
Richard H. Donohue Jr., 33, was shot and wounded in the incident, a transit police spokesman said. CNN affiliate WBZ reported he has been with the department for three years. He was taken to Mount Auburn Hospital, WBZ said. The officer's condition was not immediately known.
Ultimately, one bomber -- 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 the law enforcement source.
Tamerlan Tsarnaev was critically injured. He had bullet wounds and an injury from an explosion. He was wearing explosives and had an explosive trigger, a source told CNN. He later died, while authorities say his brother, Dzhokar Tsarnaev, 19, escaped.
After a hellish week in Boston, residents probably didn't need to be reminded that he was believed armed and dangerous.
Warnings to Watertown residents followed. First, Boston police tweeted, "There is an active incident ongoing in Watertown. Residents in that area are advised to remain in their homes. More details when available."
Then, a warning by state police that the search was intensifying: "Police will be going door by door, street by street, in and around Watertown. Police will be clearly identified. It is a fluid situation."
Officers locked down the streets of a Watertown neighborhood -- the intersection of Dexter and Laurel streets lies in a heavily residential area -- after isolating the vehicle. SWAT members arrived on the scene, and police in full body armor carrying assault rifles ran down the streets, according to CNN affiliate WCVB.
Homeland Security Investigations agents were among those responding, a department spokesman said.
Police requested that residents in the area turn off their mobile phones, saying they believe the devices were used to detonate the Boston Marathon explosives. They went door to door, leaving an already-on-edge suburb even more anxious.
In Cambridge, police converged on another home, telling residents near Norfolk Street, "Ongoing investigation. Potentially dangerous. Stay clear."
Chris Howes, who asked local police via Twitter whether they had any advice for residents who lived on Norfolk, also wrote on Twitter, "This is scary as f***." Asked via Twitter whether he could provide more information, he tweeted, "don't really have any info. CPD told us to stay indoors so we just hear lots of sirens outside."
As news emerged that the suspects lived with their parents in the Norfolk Street area, nearby resident Amy McConnell tweeted, "Just so weird to think those guys have been walking around my neighborhood all week." She also declined to speak to CNN.
At the corner of Norfolk and Cambridge streets, police tape lined the road, blocking off Norfolk a few houses down from the white clapboard home where the suspects reportedly lived with their parents. Police and FBI buzzed around the home.
Joey Barbaso, 50, has lived in the neighborhood since he was 5. He didn't know the suspects, had never seen them.
The construction worker's pants were worn and stained with paint. The neighborhood, nestled between Harvard and MIT, is a mix of working class and college students, he said, standing in the doorway of a shop.
"You never know who you're living next to," he said. "I think it's nuts. What's this world coming to? I tell you, the world's getting screwed up more and more and more."
Meanwhile, the entire city was shutting down. The transit authority said all modes of transport -- including rail, subway, buses and ferries -- would be suspended until further notice. Taxi service in Boston was suspended.
MIT and Harvard canceled classes, as did other local colleges Boston Public Schools.
While police requested residents of Boston and all its suburbs remain at home, the Massachusetts State Police specifically singled out Boston, Watertown, Cambridge, Allston-Brighton, Belmont and Newtown. The governor's office added Waltham to the list of places where people should "stay indoors with your doors locked."