Thousands of police officers are searching in and around the northeastern U.S. city of Boston, Massachusetts, for a suspect in Monday's Boston Marathon bombings.
The other suspect was killed in an overnight shootout with police in the Boston suburb of Watertown.
Law enforcement officials say the two suspects, identified as brothers, are U.S. residents of Chechen origin.
Police converged Friday on the towns of Watertown and Cambridge searching for the 19-year-old man identified as the "white hat" suspect from video taken before the bombings.
The door-to-door search has paralyzed the Boston area. Residents have been urged to stay in their homes with the doors locked, public transit service has been suspended, and schools are closed. Airspace over Boston has been shut down.
Massachusetts Governor Deval Patrick said the situation is "serious," and asked the public to take it seriously. He said the suspect on the loose is "armed and dangerous."
Boston police identified the suspect at large as 19-year-old Dzhokhar Tsarnaev of Cambridge. The deceased suspect is his brother, 26-year-old Tamerlan Tsarnaev.
A man who said he is an uncle of the suspects, Ruslan Tsarni, said their family is ethnically Chechen and Muslim. Tsarni spoke to reporters Friday outside his home in the eastern state of Maryland. He said the brothers are "losers" if they are guilty of the bombings.
Chechnya has been plagued by an Islamic insurgency stemming from separatist wars.
In Washington, U.S. Secretary of State John Kerry refused to speculate on the implications of the brothers' Chechen roots. He said terror in any country is "unacceptable."
U.S. President Barack Obama met with his national security team Friday and was briefed on the ongoing developments in Boston.
Tamerlan Tsarnaev was fatally wounded in a gunfight with police late Thursday in Watertown.
On Thursday, the FBI released surveillance pictures and videotape of the two suspects taken Monday around the time of the Boston bombings. They labeled the suspect in a black baseball cap "Suspect One," and the suspect in a white baseball cap "Suspect Two."
A few hours later, the suspects killed a security officer at a local college, Massachusetts Institute of Technology, and stole a car at gunpoint from a victim who they kept in the vehicle for a half-hour.
Police received reports of the theft and followed the suspects into Watertown.
Police say explosives were thrown out of the suspects' car in Watertown and shots were exchanged, noting one of the suspects was seriously wounded and later died at a hospital. Police say the other suspect escaped.
Monday's twin bombings near the finish of the Boston Marathon killed three people, including an 8-year-old boy and a Chinese-born graduate student. More than 170 people were injured. Some of them lost their legs.
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