The two bomb suspects were brothers, according to their uncle

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The men are brothers from the Russian region near Chechnya – an area plagued by an Islamic insurgency stemming from separatist wars – US law enforcement officials and the uncle of the men have said. The surviving suspect has been identified as 19-year-old Dzhokhar A. Tsarnaev, while the man killed was named as Tamerlan Tsarnaev. The uncle, Ruslan Tsarni of Montgomery Village, Maryland, said that the men lived together near Boston and have been in the United States for about a decade.



He urged his nephew to turn himself in, saying: "Dzhokhar, if you are alive, turn yourself in and ask for forgiveness." Both men had international ties with military experience, according to reports. "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," said Boston Police Commissioner Edward Davis. "There is a terrorist on the loose who wants to kill people - do not open your door," officials warned. The city has effectively been shut down with residents confined to their houses and all transport having been suspended.

Residents are being greeted by the sight of armed officers outside their homes/ Shawna England

SWAT teams enter a suburban neighborhood to search for the remaining suspect

The suspect police are looking for in Watertown is the one seen in images released by the FBI of a man wearing a white cap, Davis said. "One suspect dead. One at large. Armed and dangerous. White hat suspect at large," Davis tweeted.



The other suspect in those images was killed during a pursuit by officers "with a bomb strapped to his chest," according to reports.



The suspects' father Anzor Tsarnaev described his youngest son as a 'true angel' in an interview with The Associated Press (AP).



In a telephone from the Russian city of Makhachkala, he said: "Dzhokhar is a second-year medical student in the U.S. He is such an intelligent boy. We expected him to come on holidays here." Mr Tsarnaev added that he felt his sons had been 'framed' and was convinced of their innocence.



He said: "They were set up, they were set up!



"I saw it on television; they killed my older son Tamerlan." Doctors at Beth Israel Deaconess Medical Center in Boston where a suspect in the marathon bombings was taken and later died said they treated a man with a possible blast injury and multiple gunshot wounds. FBI agents, national guard, K-9 units and SWAT teams stormed Watertown after a reported car chase involving two suspects in a hijacked car. According to eyewitness reports, the two young men then engaged in a furious gun battle with officers.

Police officers keep a man on the ground in Watertow

A police officer points his rifle at a man (not pictured) on the ground in Watertown

As police converged on the neighborhood outside Boston, multiple gunshots and explosions could be heard, available to watch in video footage below. Resident Andrew Kitzenberg, 29, said he looked out of this third floor window to see two men of slight build in jackets engaged in “constant gunfire” with police officers. As a police SUV "drove towards the shooters" it was fired upon until it was damaged and rolled to the side of the road, crashing into two cars in his driveway, he added. The two shooters, he said, had a "large and unwieldy bomb." “They lit it, still in the middle of the gunfire, and threw it," he told The New York Times.

Boston Marathon bombing suspect Dzhokhar Tsarnaev, 19, is still on the run from police

Police say the suspect in the fatal shooting of an MIT police officer is tied to the Boston bombing

State police spokesman David Procopio said, "The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers."



Boston cab driver Imran Saif said he was standing on a street corner at a police barricade across from a diner when he heard an explosion.



"I heard a loud boom and then a rapid succession of pop, pop, pop," he said. "It sounded like automatic weapons. And then I heard the second explosion."



He said he could smell something burning and advanced to check it out but area residents at their windows yelled at him, "Hey, it's gunfire! Don't go that way!"

Boston Marathon bombing suspect Tamerlan Tsarnaev was killed in a gunfight with police

Police are still conducting a door-to-door, street-by-street search due to what it called a fluid situation – the area is considered to still be extremely dangerous. Authorities urged residents in Watertown, Newton, Waltham, Belmont, Cambridge and the Allston-Brighton neighborhoods of Boston to stay indoors, while all mass transit has been shut down. Dramatically, the Federal Aviation Administration also closed airspace over a 7-mile-wide stretch of the northwest Boston area today to provide a "safe environment for law-enforcement activities" as police searched for a second Boston Marathon bomb suspect. The restriction, which echoes similar measures imposed after Monday's bombing, applies to all aircraft flying below 3,000 feet (914 metres), according to an FAA bulletin. The restriction, which echoes similar measures imposed after Monday's bombing, applies to all aircraft flying below 3,000 feet (914 metres), according to an FAA bulletin.

Officers wearing tactical gear arrive at the Watertown neighborhood of Boston

Police vehicles search neighborhoods yard by yard after a police chase and shootout

A police officer for the Massachusetts Institute of Technology was shot to death on the campus

Reporters have also been urged to move away from the scene, with police officers reportedly stating: "If you want to live, turn off your cell phone," - fearful that explosives could be remotely detonated. Authorities could then be heard shouting for somebody to get on the ground and put their hands up and a loud thud was heard after someone shouted "fire in the hole." Earlier, a police officer for the Massachusetts Institute of Technology (MIT) was shot dead at the prestigious university's Cambridge campus, sparking a manhunt in a community on edge just days after the Boston Marathon bombing. The officer was responding to a report of a disturbance when he was shot multiple times.

The location and time line of the suspects in the Boston Marathon explosions

The officer, who was not immediately identified, was taken to a hospital and was pronounced dead from his wounds. The university warned students in an emergency alert to stay indoors and said that one building on campus had been surrounded by police. The shoot-out came hours after the FBI released images and video of the Boston bombing suspects as they launched a hunt for two "armed and extremely dangerous" men.

Gunshots were heard near a building on the campus of the Massachusetts Institute of Technology