Residents of Boston and surrounding suburbs have been told to stay indoors amid a massive police manhunt for one of two brothers suspected of carrying out the Boston Marathon bombings on Monday.

The younger brother was the suspect seen wearing a white cap backward in video and photos released by the Federal Bureau of Investigation on Thursday. The release prompted a large number of tips from the public, federal officials said. The older brother was wearing a black cap in the video and photos.

Dzhokhar Tsarnaev, 19, escaped a shootout and mounted house-to-house search where his brother, Tamerlan Tsarneav, 26 had been killedafter a bloody night of shooting and explosions in the streets.

He is a student at the University of Massachusetts, Dartmouth, university spokesman Robert Lamontagne said.

Three people died and more than 170 were hurt when two bombs exploded near the finish line of Monday's marathon.

The FBI released several images of two men they were hunting in relation to the bombing, one wearing a white cap, the other a black cap.

Police said "suspect number one" had been killed early on Friday, and they were looking for "suspect number two", the "white-capped individual", later named as Dzhokhar Tsarnaev.

Reports said the black-capped suspect - widely named in the US media as Tamerlan Tsarnaev - was the fugitive's elder brother.

Both are said to be of Chechen origin, and are reported to have moved to the United States about 10 years ago.

The violence began at around 10:30 p.m. Thursday with the robbery of a 7-Eleven in nearby Cambridge, authorities said. The two men then fatally shot an MIT campus police officer and carjacked a Mercedes sport-utility vehicle at gunpoint, keeping the vehicle's owner hostage for about a half-hour, police said. The owner was released at a gas station in Cambridge, authorities said. He wasn't injured.

As police pursued the vehicle, explosive devices were thrown from the car, authorities said. "There was an exchange of gunfire" between police and the suspects," Mr. Alben said.

Hundreds of police officers descended on the Cambridge and Watertown areas as the violence unfolded Thursday night, authorities said. Residents said they heard loud explosions and gunfire.

In the Boston suburb of Watertown, officers and the men were involved in a gun battle lasting 10 minutes, according to witnesses.

Katie Blouin, 24 years old, of Watertown, said FBI agents and local police entered her house, searching before telling her boyfriend to lock the house's doors.

"I'm shaking," she said. "It just makes you so nervous."

Adonis Karageorgis, a 35-year-old dental student who lives in Watertown, said he heard a loud explosion from his apartment balcony

"I looked up and saw the sky light up," he said. "You could smell the smoke."

Beth Israel Deaconess Medical Center geared up for a potential mass-casualty event when one doctor there—who lives near the scene of the gunfight—heard the commotion outside his home.

"When I started hearing the gunshots and explosions, given what had happened over at MIT and seeing all the police cars rushing into Watertown and past my house and hearing all the sirens, I knew or felt very strongly that this was related to the events from earlier this week as well as from what happened over at MIT," said David Schoenfeld, an emergency physician there, during a news conference early Friday.

"Because of that, I felt as though something large enough was going on in the community that it warranted calling the emergency department and coming in," he said.

The MIT campus police officer that was killed wasn't identified. The officer had multiple gunshot wounds and was taken to a nearby hospital, where he was pronounced dead, according to a statement on the Middlesex County District Attorney's website.

Shortly before midnight Thursday, police were gathered in Watertown, and a stretch of the campus near Vassar Street and Main Street in Cambridge was cordoned off. Police were searching through woods with dogs and flashlights.

Dozens of police officers gathered at Massachusetts General Hospital, where the injured officer was reportedly taken. Officers directing traffic asked those who arrived in a panic: Some Police Officers wept openly as they hurried into the emergency room.

The authorities in Massachusetts Bay have suspended the transport system and no vehicles are being allowed in or out of the Watertown area.

The warning to stay indoors was later extended to the whole of Boston, in what correspondents said was an unprecedented move.

"Stay indoors with the doors locked, and do not open the door for anyone other than a properly identified law enforcement officer," Massachusetts Governor Deval Patrick said on Friday morning.

Overnight, video footage emerged showing a fully-clothed suspect lying on the floor, surrounded by police. More video was shown by US media of a suspect being led into a police car after being stripped of his clothes.

But it is not clear who the men were, or what happened after their apparent arrests. Boston police Commissioner Ed Davis said he believed the man being hunted in Watertown was a "terrorist".

"We believe this to be a man who came here to kill people," he said.

Monday's attack on the Boston Marathon killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.