US law enforcement officers were Friday conducting a massive door-to-door manhunt for a suspect behind the Boston Marathon bombings, after his accomplice was earlier killed in a shootout with police.
The two suspects are brothers, aged 19 and 20, from Chechnya and have lived in the United States for about one year as legal residents, police sources told NBC News. The broadcaster also reported that they had received "military training."
The men exchanged gunfire with police officers in the early hours of the morning in the Boston suburb of Watertown. The older brother, identified in photos released by police as a male with a black cap, was wounded in the shootout and died in hospital.
The second suspect, who in the photos appears wearing a white cap, fled the scene. Police have put the 380,000 residents of Watertown under lockdown and launched a door-to-door search for the 19-year-old.
The manhunt follows a chaotic night of violence in the Boston area: A police officer was shot dead on the campus of the Massachusetts Institute of Technology in Boston and hours later the firefight broke out in Watertown, a few kilometres west of the city.
The search centred on a 20-block area in Watertown and involved some 10,000 officers, according to reports.
The death of one of the two suspects puts more pressure on police to catch the second one alive to shed light on their identities and motives.
Massachusetts officials have shut down public transportation in and around Watertown, citing public safety concerns as police search what they called a "terrorist." Businesses have been urged to remain closed and people asked to avoid outside gatherings.
Just hours after police released their photos on Thursday night, the two suspects carjacked a vehicle in Watertown and later clashed with police.
"There was an exchange of fire between police and two suspects in this SUV," a police officer said, referring to the carjacked vehicle. "Several explosive devices were released from the vehicle at police. One suspect was arrested and another fled."
Another officer added: "We believe this to be a terrorist. We believe this is a man who has come here to kill people."
One of the officers, who spoke to reporters in Watertown, said the suspect's appearance was "consistent with the description of the white capped individual" in the photos, describing him as a "white skinned, Caucasian male."
A doctor at the Beth Israel hospital in Boston where the first suspect was brought after the shootout said he had died of multiple wounds, including injuries from a blast and gunshots.
A hospital official said: "We dont have any information on the identity of the deceased patient. It's an adult male."
One police officer was seriously wounded in the gunfight with the suspects.
The MIT campus is around three kilometres from the centre of Boston, where two bombs detonated at the finishing line of the marathon on Monday, killing three people and injuring more than 170.
The campus police officer was shot several times on the grounds of the university late on Thursday as he was responding to a disturbance. He died in hospital.
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