US police say they have captured a 19-year-old man suspected of carrying out the Boston Marathon bombings with his brother.
Dzhokhar Tsarnaev had escaped a shoot-out in which his brother Tamerlan Tsarnaev, 26, died.
The teenager was found hiding in a boat in a backyard in the Boston suburb of Watertown, US reports say.
He was seriously injured when taken into custody and is being treated at a Massachusetts hospital.
The FBI is also questioning a further two men and a woman.
Police teams and armoured personnel carriers had raced to the district and witnesses at the scene reported hearing gunfire.
Earlier on Friday armed police had been going door to door looking for the teenager, and people across the city were told to stay indoors.
The FBI has said the brothers were suspects in the deadly explosions near the Boston Marathon's finish line on Monday that killed three people and wounded at least 170.
The brothers are said to be of Chechen origin, and are reported to have moved to the United States about 10 years ago.
A US law enforcement source said the FBI interviewed the older brother, Tamerlan Tsarnaev, in 2011 at the request of a foreign government, Reuters reports.
The source said no "derogatory" information was found and the matter was closed.
A massive manhunt involving thousands of police began after a Massachusetts Institute of Technology campus officer was killed.
Late on Thursday police discovered the officer, 26-year-old Sean Collier, shot several times in his car in an apparent confrontation with the suspects, Reuters reports.
Police said the two brothers carjacked a Mercedes SUV and held the owner hostage for about half an hour before being released.
Officers chased the car into the Boston suburb of Watertown, during which time the suspects threw explosives from the car and exchanged gunfire with police. A transit police officer was hurt in the shootout.
The elder brother, Tamerlan Tsarnaev was hit during the shootout and later pronounced dead at hospital.
A huge hunt was launched for the second suspect and hundreds of police officers and FBI agents descended on Watertown.
Police carried out door-to-door searches and warned people to say indoors, train services were shut down and the city was effectively put into lockdown.
The brothers are sons of Chechen refugees from the troubled Caucasus region of southern Russia and are said to have lived in Kyrgyzstan and Dagestan before coming to the United States, the BBC reports.
The father of Dzhokhar Tsarnaev said his son was a second-year medical student in the US and was hoping to be a brain surgeon. Anzor Tsarnaev told the BBC he believed the secret services had framed his sons.
In an impassioned interview with reporters outside his Maryland home, uncle Ruslan Tsarni said he had not spoken with Dzhokhar and Tamerlan since 2009, and pleaded with Dzhokhar to turn himself in and ask forgiveness.
"He put a shame on our family, he put a shame on the entire Chechen ethnicity," he said.