Police surrounded a house in the Boston suburbs where a suspect in the deadly marathon bombings was believed to be hiding late Friday after a burst of gunfire, reports said.
Police squads and armored personnel carriers raced to Watertown, where media reports said the suspect was wounded and hiding in a boat in the backyard of a house.
At one stage a series of about 15 explosions erupted, which local television channels said could have been flash grenades. But there was no apparent end to the showdown.
Police helicopters buzzed overhead, and bomb squad vans and ambulances were in position around the house in Franklin Street in Watertown.
The Boston Globe quoted a source as saying police had 19-year-old Dzhokhar Tsarnaev "pinned down." CNN said police believed they had the suspect surrounded.
Gunfire erupted in the district only minutes after police told a press conference they believed that Tsarnaev had escaped a mass dragnet thrown over Watertown.
Chechen brothers Dzhokhar and Tamerlan Tsarnaev are the main suspects in the Boston marathon bombing on Monday, in which three people were killed and about 180 injured.
Tamerlan, 26, was killed in a shootout with police in the early hours of Friday in Watertown after an earlier gun battle in which a police officer was shot dead.
The media reports said Dzhokhar Tsarnaev was wounded in the shootout.