Gunfire has been heard in a Boston neighbourhood where the hunt continues for one of two men thought to be behind the marathon bombings.
Described as "armed and dangerous", 19-year-old Dzhokhar A Tsarnaev is being sought in Watertown, around nine miles from the Boston city centre.
He escaped an earlier shoot-out in which his 26-year-old brother and fellow suspect Tamerlan Tsarnaev was killed. Dzhokhar was said to be badly injured in the stand-off.
Footage shows emergency and military vehicles speeding through the area after the shots were heard in Franklin Street.
It was not immediately clear whether authorities had found Dzhokhar.
Some reports said he was found in a boat in a Watertown backyard, but that has not been confirmed.
Massachusetts State Police spokesman David Procopio said there was "renewed activity" in connection with the manhunt.
About 10 minutes before the muffled gunfire was heard, Boston police lifted their request that residents stay indoors.
But a short time later, police reinstated the stay-indoors order as the shots rang out.
Earlier, Colonel Timothy Alben said: "We do not have the apprehension of our suspect this afternoon, but we will have one."
The brothers moved to the US a decade ago from a Russian region near Chechnya and had been living in Cambridge, Massachusetts, as legal permanent residents.
Their father, Anzor Tsarnaev, told reporters in Russia that he believed his sons had been framed and described the killing of Tamerlan as "cowardly".
And 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.
Maret Tsarnaeva, the men's aunt, said Tamerlan had recently became a devout Muslim who prayed five times a day.
Speaking to reporters at her home in Toronto, Canada, she also said the older brother was married and had a daughter in the US.
Meanwhile, FBI agents are at the New Jersey home of the two men's sister, who police said is "very upset".
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.
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.
The shoot-out in 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 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 shoot-out in Watertown.
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.
Mr Kitzenberg 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.
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.
Several homes in Watertown suffered extensive damage from the bomb blast and were caught in the crossfire in the shoot-out.
Tamerlan Tsarnaev is said to have run at police and was shot at and arrested. He later died in hospital despite attempts to resuscitate him.
According to NBC, police ran over him 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.
A police officer was also shot in the stand-off 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.
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.
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.
Chechnya's president, Ramzan Kadyrov, said in a statement: "Any attempts to draw the link between Tsarnaevs (even if they are guilty) and Chechnya are in vain.
"They grew up in USA and their views and beliefs were formed there. One needs to seek the roots of evil in America. All the world should be fighting terrorism together.
"We know it better than anyone else. We wish all those who suffered to get well soon and we share the feeling of sorrow with Americans."