BOSTON was in virtual lockdown as a manhunt continued for one of two brothers suspected of bombing the city's marathon following separate shootings in which his sibling and a police officer died.

Dzhokhar Tsarnaev, 19, remained at large after fleeing the scene of a shootout in which Tamerlan Tsarnaev, 26, died.

Residents of the working-class suburb of Watertown locked their doors as police searched each house for the man. Controlled explosions were also heard in the neighbourhood.

Speculation mounted that the men might have had friends in the city, which has a large Armenian and Russian population.

Watertown resident Liz Yusem said she wasn't letting her two children watch tele-vision because she was worried the manhunt in their neighbourhood would terrify them.

Already, she said her daughter had expressed fear that "bad guys" were coming for them. "It's too scary for young children," she said.

The dramatic developments came after three people died, including an eight-year-old boy, and more than 170 hurt when two bombs exploded during Monday's Boston marathon.

Images showing two men were issued by the FBI on Thursday, shortly before the shootings which began at a college campus on the outskirts of Boston.

Tamerlan died after a shootout with officers that began at Massachusetts Institute of Technology. His death followed the killing of campus police officer Sean Collier, 26.

The suspects carjacked a vehicle before leading police on a chase that led to Tamerlan being shot dead. Another officer was also injured in exchanges.

The pair, who were originally from Chechnya, had moved to America a decade ago.

Fugitive Dzhokhar, who won a college scholarship and wanted to become a brain surgeon, described himself on a social networking site as a minority from a region that includes Chechnya, Dagestan and Ingushetia. A spokesman for Russian president Vladimir Putin said he condemned all terrorists regardless of their nationality.

The boys' uncle, Ruslan Tsarni, said they were losers who were unable to settle. He added that their motive was probably, "not being able to settle themselves and thereby hating everyone who did".

He said the violence, "put a shame on our family. It put a shame on the entire Chechen ethnicity".

A Russian social networking site bearing Dzhokhar Tsarnaev's name paid tribute to Islamic websites and to those Chechen independence sites.

Another uncle, Alvi Tsarnaev, said he had received a call from Tamerlan on Thursday evening just before he was shot dead in which he asked for forgiveness. Mr Tsarnaev said: "He said: 'I love you and forgive me'."

The suspects' father Anzor described fugitive Dzhokhar as "a true angel and an intelligent boy" who was studying medicine, yet claimed they had been set up by the authorities.

Hundreds of officers in SWAT gear rolled through the streets of Watertown in armoured vehicles with Black Hawk helicopters circling overhead.

Police sent out an automated call to all homes ordering citizens off the streets, leaving the city's commercial area as a police staging area.

Tensions rose further when Massachusetts officials shut down all public transit and expanded the lockdown to include the city of Boston.

Police asked people to stay inside, lock their doors and not admit anyone but properly identified police.