WATERTOWN, Mass. -- One suspect linked to the Boston Marathon bombings is dead, and the second is on the loose, armed and dangerous, police said early Friday.
Officers were going door to door in the Boston suburb of Watertown. They warned residents to stay inside and not open their doors unless an identifiable police officer was on the other side.
The suspect still at large has brown, curly hair, police said. In photographs provided by federal authorities, he is wearing a white hat. Officials said the suspect is directly linked to Monday's marathon bombings, which left three dead and more than 170 injured.
"We believe this to be a terrorist," Boston Police Commissioner Ed Davis told reporters early Friday. "We believe this to be a man who's come here to kill people. We need to get him in custody."
Another police spokesman, Col. Timothy P. Alben, told the news briefing: "What we are looking for right now is a suspect consistent with the description of the white-capped individual that was involved in the bombing of Monday's Boston Marathon."
The night that resulted in the death of one suspect and the search for the second began several hours before and several miles to the east, at the Massachusetts Institute of Technology. A campus police officer was found fatally shot there in his car late Thursday night, officials said.
Authorities said the officer had been shot multiple times. He was transported to Massachusetts General Hospital and pronounced dead.
A short time later, police received reports of an armed carjacking by two males in a nearby area of Cambridge. As the police gave chase to the stolen Mercedes SUV, the suspects in the vehicle began throwing "explosive devices" from the car, according to the Middlesex district attorney's office.
Police and the suspects shot at each other in a Watertown neighborhood, an exchange that seriously wounded a transit officer, officials said. The officer's condition was described as critical.
"Shots fired!" one officer said over a police scanner. "Explosives, some kind of grenades, they're in between the houses down there."
One officer worried that cellphones could set off “one of these devices.”
“Units in the area: Power down all cellphones for fear of explosion. Power down all cellphones,” the scanner said.
Ambulances congregated at Arsenal Mall in Watertown as police cars sped by. Police with dogs searched the area. A chopper hovered overhead and the air smelled smoky. Bomb squad vehicles packed the residential neighborhood about four miles from MIT.
A Watertown man who lives about a mile from the cordoned-off area, Aaron Janikian, 27, said he was smoking outside his house when he heard “distinct gunshots” and “a big boom separate from the gunshots.”
That was followed, he said, by “more sirens than I’ve ever heard in my life.”
He drove over to see what was happening. FBI, Boston police and SWAT teams had responded. Janikian said he heard two agents wearing FBI jackets say, “IED” -- an apparent reference to an improvised explosive device.
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Times staff writers Andrew Tangel in Cambridge, Kim Murphy in Seattle, and Matt Pearce and Marisa Gerber in Los Angeles contributed to this report.