Police are frantically searching a Boston suburb for a suspect in Monday's marathon bombing after a bloody night of shooting saw a second suspect and a police officer killed.
The two suspects killed a university police officer, injured a transport officer in a firefight and threw explosive devices at police during a getaway attempt in the early hours of the morning across the towns of Cambridge and Waterdown, west of the city still recovering from this week's terrorist attack that killed three people and left over 150 injured.
The missing suspect has been identified as 19-year-old Dzhokhar A Tsarnaev from Cambridge. National security officials have confirmed that the two suspects are brothers; their uncle has confirmed the suspect killed by police was Tamerlan Tsarnaev, 26.
Ruslan Tsarni of Montgomery Village, Maryland, told The Associated Press that the men lived together near Boston and have been in the United States for about a decade. They travelled here together from a Russian region near Chechnya.
Dzhokhar was born in Kyrgyzstan and has a Massachusetts driver's license, according to officials cited by NBC News. Tamerlan is believed to have been born in Russia.
Boston police commissioner Ed Davis confirmed that Dzhokhar is still at large in Watertown, a town about 6 miles north-west of Boston. Swat teams could be seen approaching houses in Watertown as the sun rose this morning as they conducted a house-to-house search across a 20-block perimeter.
The Boston Globe reports officers dragging a handcuffed woman out of the address the suspects were believed to be living in.
The whole of Boston is under a 'shelter in place' order. Everyone has been asked to stay indoors.
Mr Davis earlier described Dzhokhar Tsarnaev as the 'white hat suspect,' referring to a man seen in video footage released by the FBI late last night of people they believed to be involved in the marathon bombing.
In a news conference, Mr Davis said: "We believe this to be a terrorist. We believe this to be a man who has come here to kill people. We need to get him in custody."
Police say Dzhokhar Tsarnaev is believed to be wearing a grey hooded sweatshirt. He is white with dark hair and appears to be in his twenties.
Colonel Timothy Alben, superintendent of Massachusetts State Police said: "We believe these are the same individuals that were responsible for the bombing on Monday of the Boston marathon. We believe they are responsible for the death of an MIT police officer and the shooting of an MBTA officer."
Locals are being warned to be extra vigilant after reports of a suspicious device by staff at the Massachusetts Institute of Technology (MIT) in Cambridge. The area is in lock-down, as authorities order businesses not to open and all traffic to be stopped while the operation continues.
The sequence of events began late last night with the armed robbery of a 7-11 in Cambridge, followed by reports of loud shooting on the campus of the Massachusetts Institute of Technology (MIT). A campus police officer was shot after investigating a disturbance in the area and later died in hospital.
Minutes later a black Mercedes was carjacked from Kendall Square, close to the shootout, leading police on a chase through Watertown.
The chase led officers through a residential neighbourhood, where a shootout ensued between officers and the two suspects.
Police confirmed that explosive devices were thrown at officers from the vehicle the suspects were traveling in.
In the ensuing firefight, one suspect was shot while the other managed to flee.
A transport police officer, identified as 33-year-old Richard Donohue Jnr, was also hit and is currently in surgery at a Cambridge hospital.
Pictures posted on Twitter by witnesses in the residential area showed a man with a large backpack standing off with police.
The injured suspect was later pronounced dead in hospital.
Officials at Beth Israel Hospital in Boston said a man was brought in heavily guarded by police and was already under "traumatic arrest". Staff said that the man's injuries were consistent with multiple gunshot wounds and a blast of some sort, possibly from an explosive device.
Dr Richard Wolfe, chief of emergency medicine at the hospital, said that the man's injuries were "more than just gunshot wounds".
Hospital staff refused to give details about the man's identity.
Watertown was in lockdown this morning, with hundreds, and possibly thousands, of police, FBI, bomb squad and national guard personnel combing the town's streets for the second Boston marathon bomb suspect. At the eastern end, a police cordon was up around the Coolidge Square area.
Kris Hagopian lives inside the exclusion zone. Last night, he was at his mother's home just beyond the cordon when he saw the news break on TV. His wife and children are still inside the closed-off area, in their home, as is his sister and her family.
On Monday, Mr Hagopian says, standing outside the 7-11 just beyond the cordon, he was near the marathon finish line on Boston's Boylston sSreet. “Now, look, its come to my town,” he said.
On his phone, he showed the texts messages he'd sent to his son, asking his family to bolt the doors and windows.
“This is a small community, mostly Armenians and Greeks. Everyone knows everyone by name,” he said, pointing in the direction of the exclusion zone.
“I own a liquor store in there,” he said, waiting beyond the police line at 6 am.
Massachusetts State Police warned people in the suburb to stay inside as they conduct a door-to-door, street-by-street search.
All public transport in Boston has been suspended and all classes at MIT cancelled. Harvard University, also in Cambridge, will also be closed.
Federal officials last night had significantly widened the investigation into the bombing by releasing photos and a video showing the men near the site of the explosions.
Richard DesLauriers, the special agent in charge of the Federal Bureau of Investigation's Boston office, said he considered the two suspects “armed and extremely dangerous” and asked the public to pass on any tips to law enforcement officials.
The first suspect was observed wearing a black cap, while the second one was seen wearing a white cap, Mr DesLauriers said. Both appeared to be carrying bags on their backs. According to the FBI, the second suspect placed his bag at the site of the second of the two blasts that struck the marathon on Monday.
The explosions killed three people and wounded more than 170 near the marathon's finish line on Boston's Boylston Street. The youngest fatality was 8-year old Martin Richard from the nearby community of Dorchester. 29-year old Kyrstle Campbell and 23-year old Lu Lingzi, a Boston University graduate student from China, were also killed by the blasts.
“Today we are enlisting the public's help to identify the two suspects... Somebody out there knows these individuals,” Mr DesLauriers said.
The developments came just hours after President Obama and first lady Michelle Obama attended a special service for the victims at Boston's Cathedral of the Holy Cross.
Tamerlan Tsarnaev (left), 26, and Dzhokhar Tsarnaev, 19
