Police killed one suspect in the Boston Marathon bombing in a shootout and mounted house-to-house searches for a second man on Friday, with much of the city under virtual lockdown after a bloody night of shooting and explosions in the streets.
Authorities cordoned off a section of the suburb of Watertown and told residents not to leave their homes or answer the door as officers in combat gear scoured a 20-block area for the missing man, who was described as armed and dangerous.
Officials identified the hunted man as Dzhokar A. Tsarnaev, 19, and said the dead suspect was his brother, Tamerlan Tsarnaev, 26.
The fugitive described himself on a social network site as a minority from southern Russia's Caucasus, which includes Chechnya, Dagestan, Ingushetia and other predominately Muslim regions that have seen two decades of unrest since the fall of the Soviet Union.
Boston came to a virtual standstill after authorities urged everyone to stay at home. Public transportation throughout the metropolitan area was suspended, and air space was restricted. Universities including Harvard and M.I.T. and public schools were closed.
During the night a university police officer was killed, a transit police officer was wounded, and the suspects carjacked a vehicle before leading police on a chase that led to one suspect being shot dead.
Police were searching for the younger Tsarnaev, previously known only as Suspect 2, who was shown wearing a white cap in surveillance pictures taken shortly before Monday's explosions and released by the FBI on Thursday.
"We believe this to be a terrorist," said Boston Police Commissioner Ed Davis. "We believe this to be a man who has come here to kill people. We need to get him in custody."
The older brother, previously known as Suspect 1, who was seen wearing a dark cap and sunglasses in the FBI images, was pronounced dead.
The FBI on Thursday identified the men as suspects in the twin blasts believed caused by bombs in pressure cookers placed inside backpacks left near the finish line of the Boston Marathon. The blasts killed three people and wounded 176 in the worst attack on U.S. soil since the suicide hijacking attacks of Sept. 11, 2001.
The brothers had been in the United States for several years.
A Russian language social networking site bearing Dzhokar Tsarnaev's name paid tribute to Islamic websites and to those calling for Chechen independence. The author identified himself as a 2011 graduate of Cambridge Rindge and Latin School, a public school in Cambridge, Massachusetts.
He said he went to primary school in Makhachkala, capital of Dagestan, a province in Russia that borders on Chechnya, and listed his languages as English, Russian and Chechen.
He posted links to videos of fighters in the Syrian civil war and to Islamic web pages with titles such as "Salamworld, my religion is Islam" and "There is no God but Allah, let that ring out in our hearts."
