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Police were still focusing on a small section of Boston in their hunt for the second Boston Marathon bomber continued following a chaotic night of mayhem that saw a police officer and one of the suspects killed. For most of the morning and early afternoon, the search was concentrated on the city's Watertown section, where Black Hawk helicopters were patrolling the sky and police were going door-to-door hunting for Dzhokhar Tsarnaev. Police say he and his older brother, Tamerlan Tsarnaev, placed the bombs that killed three and injured 176 at Monday's race. 
The sibling suspects were Chechen brothers, apparently radical Muslims from Dagestan, a province in Russia that borders Chechnya. Tsarnaev, 19, of Cambridge, Mass.,  is believed to be armed and extremely dangerous, and authorities were telling people in much of Boston to remain indoors.
"We have several other new leads that have just developed with the past few minutes," said Massachusetts State Police Police Superintendent Timothy Alben, speaking at a news conference early Friday afternoon. He said police had covered "60 to 70 percent" of the perimeter, inside which they believe Tsarnaev is hiding.
Police believe Tsarnaev could be armed and consider him extremely dangerous. He and his brother — in a vehicle they carjacked from a man who later escaped — led police on a chase through city streets after robbing a 7-11 in Cambridge and killing a Massachusetts Institute of Technology police officer, according to authorities.
The suspects threw explosives from the car and exchanged gunfire with police who were in pursuit as it headed into Watertown, according to the district attorney's news release. Tamerlan Tsarnaev, 26, was shot several times in the gunfight and pronounced dead late Thursday at an area hospital. But his younger brother escaped, and continued to elude authorities, who were going door-to-door all morning in Watertown.
"Suspect No. 1 is dead, Suspect 2 is on the run," Massachusetts Gov. Deval Patrick said at a Friday morning press conference. "There is a massive manhunt underway."
Police also found a car believed to be registered to Tamerlan Tsarnaev in Boston, after earlier issuing a lookout bulletin to Connecticut State police. The Black Hawk choppers were flying above Watertown "to provide extra eyes," sources told The Boston Globe.
During the pursuit, a MBTA transit police officer was seriously injured and transported to the hospital, according to a news release. He was identified as Richard H. Donahue Jr., 33, and was being treated at Mt. Auburn Hospital.
Schools are closed, train and bus service is suspended and police were telling residents of neighborhoods including Cambridge, Waltham, Watertown, Newton, Arlington and Belmont to stay indoors.
The suspects apparently surfaced just hours after the FBI released their imaged late Thursday afternoon, going on a bloody rampage that claimed the life of MIT Police Officer Sean Collier, 26. He was found shot to death in his squad car at 10:20 p.m. Thursday. It was not clear if he was killed before or after the convenience store was robbed, at about 10 p.m.
Moments after the shooting, the brothers carjacked a Mercedes SUV from Third Street in Cambridge and forced the driver to stop at several bank machines to withdraw money. The driver, who managed to get away, told police that the brothers had bragged to him that they were the marathon bombers, law enforcement authorities said.
“The guy was very lucky that they let him go,” Massachusetts State Police spokesman David Procopio said.
As police were working to activate the tracking device on the stolen SUV, other patrol officers spotted it in nearby Watertown, touching off the dramatic chase.
Procopio said the shooting took place about 10:30 p.m. outside an MIT building. The area was cordoned off and surrounded by responding law enforcement agencies, according to a posting on the university's website.
Earlier Friday, Cambridge police and the Middlesex District Attorney's office said the MIT officer was responding to a report of a disturbance when he was shot multiple times late Thursday. He later died at a hospital. His name was not immediately released.
The shooting came little more than three days after the twin bombings on the Boston Marathon that killed three people, wounded more than 180 others and led to an increase in security across the city.