Bomb suspect hunt turns Boston into battle zone

Boston was a city under siege Friday, as more than 9,000 police backed by helicopters and armored vehicles hunted for a teenager wanted for the deadly marathon bombings.

The search for 19-year-old Dzhokhar Tsarnaev left hundreds of thousands of people cowering in their homes after a chaotic night of gunfire and explosions turned one of America's major metropolitan areas into a battle zone.

Jonathan Crespo said he was watching "Zero Dark Thirty", the film about the hunt for Osama bin Laden, in his home in the Boston suburb of Watertown when Tsarnaev and his brother Tamerlan passed by at the start of their rampage.

"My fiancee thought she heard police cars. I thought she was tripped out by the movie. But I saw numerous vehicles, I did hear some kind of an explosion -- probably a flash-bang grenade or something," Crespo said.

Shortly afterwards, Tamerlan Tsarnaev -- the man identified as Suspect One by the Federal Bureau of Investigation (FBI) -- was dead following a shootout with police in the town. One police officer was wounded.

"The explosions woke me up, basically. It was pretty scary. I wasn't expecting anything like that," said Crespo.

Another resident, Yvonne Alaykib, told of hearing gunshots and machine guns in the dark. "It was like a fight -- it was back and forth, pretty scary," she said.

The pair of suspects had shot dead a police officer earlier on the Massachusetts Institute of Technology campus, where some students locked themselves in a laboratory with the lights off for three hours.

"We heard a lot of rumors," said a woman graduate student, who only gave her name as Pallavi. "We've just been sitting inside in the dark, locked doors, texting, hearing whatever rumors we could from our friends."

A doctor, David Schoenfeld, who lives in Watertown, heard the explosions and decided to go to work at the Beth Israel hospital. He got there in time to treat 26-year-old Tamerlan Tsarnaev at the emergency unit.

"When the patient arrived, he was in critical condition," Schoenfeld told reporters. The injuries he suffered from gunshots and explosives were too much to overcome.

On Friday, thousands of police surrounded Watertown, a normally quiet town of 33,000 people and home to the Tsarnaev brothers, who have been blamed for Monday's attacks on the Boston marathon, which killed three and wounded 180.

Specially-organized buses ferried frightened residents out. Police SWAT squads went from house to house looking for Dzhokhar Tsarnaev.

The hunt was concentrated on about 1.5 square miles (3.8 square kilometers) of Watertown.

A police helicopter hovered over the home in Norfolk Street where the brothers lived. Dozens of FBI agents were in the street.

Authorities halted all public bus and train services in the Boston region and told hundreds of thousands of residents to stay home behind locked doors in a bid to isolate the suspect.

Those who did not leave received robot phone calls warning them to stay indoors and only answer the door if they were sure it was a police officer.

Police were armed with shotguns and automatic rifles, and plainclothes agents had at least a handgun as they warily watched events unfold.