WATERTOWN, Mass. -- One of two suspects in the Boston Marathon bombing is dead after the killing of a university officer and a shootout with police, and a massive manhunt is underway for the other, authorities said early Friday. Police are locking down some neighborhoods in Boston and its western suburbs as they search for the remaining suspect, who is known as the man in the white hat from marathon surveillance footage. The suspects were identified to The Associated Press as coming from the Russian region near Chechnya, which has been plagued by an Islamic insurgency stemming from separatist wars. A law enforcement intelligence bulletin obtained by the AP identified the surviving bomb suspect as Dzhokhar A. Tsarnaev, a 19-year-old who had been living in Cambridge, just outside Boston, and said he ''may be armed and dangerous.'' The dead suspect is described as Tsarnaev's 20-year-old brother, said the official, who declined to be identified because he is not authorized to comment publicly. Authorities urged residents in Watertown, Newton, Waltham, Belmont, Cambridge and the Allston-Brighton neighborhoods of Boston to stay indoors. All mass transit was shut down and businesses were asked not to open Friday. People waiting at bus and subway stops were told to go home. "We believe this to be a terrorist," said Boston Police Commissioner Ed David of the suspect on the loose. "We believe this to be a man who's come here to kill people. We need to get him in custody." The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus in Cambridge late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed. Hours earlier, police had released photos of the marathon bombing suspects and asked for the public's help finding them. A new photo of the suspect on the loose was released later showing him in a grey hoodie sweatshirt. It was taken at a 7-11 in Cambridge, just across the Charles River from Boston. The first images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the bombing victims.