Armed police are hunting one of two men thought to be behind the Boston Marathon bombings, after shooting dead his alleged accomplice.
Described as "armed and dangerous", 19-year-old Dzhokhar A Tsarnaev is being sought by hundreds of officers in Watertown, after escaping the shoot-out in which his 26-year-old brother Tamerlan Tsarnaev was killed.
At least one gun shot was heard shortly before 4pm UK time in the town, which lies around nine miles from Boston city centre.
The suspect being hunted by police
The pair moved to the US a decade ago from a Russian region near Chechnya and had been living in Cambridge, Massachusetts, as legal permanent residents.
Speaking to reporters, the suspects' uncle Ruslan Tsarni issued an appeal to Dzhokhar Tsarnaev, saying: "Turn yourself in, and ask forgiveness."
Mr Tsarni, who lives in Maryland, said he had not seen the pair for several years. He said the family was ashamed and that he loves and respects the US.
Sean Collier, the MIT police officer who was shot and killed
The younger brother is said to have posted links to Islamic websites and others on a Russian-language social networking site, calling for Chechen independence .
All public transport remains suspended in Boston - and all streets and businesses in Watertown locked down - while a major house-to-house manhunt continues for the suspect who escaped.
The shoot-out in the residential suburb of Watertown between both suspects and dozens of armed officers came after Sean Collier, a 26-year-old campus policeman, was shot dead in his vehicle at the nearby Massachusetts Institute Of Technology.
The suspect at the marathon among the bombing victims (Pic: Kevin O'Keefe)
The suspects are believed to have killed the MIT campus policeman before carjacking a black Mercedes SUV, taking its owner hostage and driving off towards Watertown with police in pursuit.
On their way, the hostage was dumped at a petrol station after around 30 minutes, before the shootout ensued in Watertown.
Boston Police commissioner Ed Davis said: "What we are looking for right now is a suspect consistent with the description of suspect number two - the white-capped individual who was involved in Monday's bombing of the Boston Marathon."
He was described as light-skinned with brown wavy hair and dressed in a grey hooded top.
Mr Davis said: "You have seen the picture. You all have it. That's the individual we are looking for at this moment.
The dead suspect, who ran at officers during the stand-off
"We believe this to be a terrorist. We believe this to be a man who came here to kill people."
"We believe these to be the same individuals who are responsible for the bombing on Monday at the Boston Marathon."
Massachusetts Governor Deval Patrick said a "massive manhunt" was under way.
"This is a serious situation, we are taking it seriously and we are asking the public to take it seriously," he said.
A Swat team on a roof in Watertown (Pic: Samantha England)
Officials said the operation could take hours and asked the public to be patient.
The New York Times quoted Watertown resident Andrew Kitzenberg, who said he saw the pair, wearing backpacks, shooting at dozens of police officers from behind a black Mercedes SUV.
The 29-year-old said the officers and the men were 70 yards apart and engaged in "constant gunfire".
He said an explosive device was also thrown at the police and went off some 20 yards from where they were shooting.
Boston Police commissioner Ed Davis holding a press conference in Watertown
One suspect is said to have run at police and was shot at and apprehended. He later died in hospital despite attempts to resuscitate him.
According to NBC, police ran over the suspect because he was wearing an IED (improvised explosive device). However this has not been confirmed by police or other officials.
He arrived at Beth Israel Deaconess Medical Centre with multiple injuries mainly around his torso including gunshot wounds, burns consistent with a blast and pieces of shrapnel all over his body.
Watertown Residents heard the gun battle and an explosion
The other, who is said to be badly injured, managed to get away from the stand-off - in which a police officer was also shot and is undergoing surgery at a Boston hospital.
Some 12 victims from the Boston Marathon blasts remain at the same hospital, one in a critical condition. Around 24 others who suffered injuries have been discharged.
Police officers arriving at the scene in the neighbourhood
A spokesman said though the hospital was open as normal, it was restricting access to "ensure the safety of patients, families and visitors", and urged people to show patience when arriving at the site.
One medic, who lives in Watertown, heard the shoot-out from his home and alerted emergency services before rushing to the hospital where he tried to revive the suspect.
Hundreds of thousands of residents across Boston and Watertown have been told to stay indoors and remain vigilant until further notice while police conduct house-to-house searches.
Several homes in Watertown suffered extensive damage from the bomb blast and were caught in the crossfire in the shoot-out.
Bullet holes through the wall and chair of one home caught in the crossfire
Police have sealed off the area and are conducting door-to-door searches and enquiries.
Sky's US correspondent Dominic Waghorn in Watertown, said: "It has been a terrifying night for the people of Watertown, and it remains so until this man is found."
Meanwhile, police converged on what is believed to be the suspects' apartment in Cambridge and a woman was reportedly dragged from the block of flats and handcuffed before she was led away.
The developments come just days after two bombs exploded near the finish line of the Boston marathon, killing three people and injuring more than 180 others.
The FBI had earlier released pictures and video of two suspects in the deadly Boston marathon bombings.
The shootout took place in the Watertown neighbourhood near Boston