Police were donning bullet-proof vests and going door-to-door as a perimeter tightened in the Watertown section of Boston, where the second Boston Marathon bomber could be hiding.
Earlier, a bullet -riddled SUV was recovered in the area following a chaotic night of mayhem that included the murder of a Massachusetts Institute of Technology police officer and a shootout with police, authorities said early Friday. Police believe the two suspects from Monday's terror attack are Chechen brothers, Muslims possibly from Dagestan, a province in Russia that borders Chechnya. The man on the loose was identified as Dzhokhar A. Tsarnaev, 19, of Cambridge, Mass. They are believed to have been here for 'several years,' sources said.
Police believe Tsarnaev could be armed and consider him extremely dangerous.
The suspects threw explosives from the car as police followed it into Watertown, according to the district attorney's news release. The suspects and police exchanged gunfire, and one of the suspects was critically injured and later died early Friday while the other escaped.
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. Police have formed a wide perimeter and believe the suspect is on foot, armed and dangerous, inside.
"Suspect No. 1 is dead, Suspect Two is on the run," Massachusetts Gov. Deval Patrick said at a Friday morning press conference. "There is a massive manhunt underway."
The dead suspect, Tamerlan Tsarnaev, 26, was shot multiple times in a gunfight with police Thursday night and pronounced dead at a hospital. Police also found a car believed to be registered to Tamerlan Tsarnaev in Boston, after earlier issuing a lookout bulletin to Connecticut State police.
Police in Maryland surrounded a home in an upscale suburb Montgomery Village, where the suspects' uncle, Ruslan Tsarni, is believed to live.
The suspects apparently surfaced just hours after the FBI released their imaged late Thursday afternoon, shooting the police officer, robbing a convenience store, carjacking a man who later escaped and engaging in a wild shootout with Boston police, in which they hurled explosives from their stolen car.
The MIT police officer who was killed was identified as Sean Collier, 26, who was shot around 10:30 p.m. Thursday.
“The MIT Police serve all of us at the Institute with great dignity, honor and dedication,” Israel Ruiz, MIT’s executive vice president and treasurer, said in a statement. “Everyone here -- those who knew Officer Collier, and those who did not -- are devastated by the events that transpired on our campus last night. We will never forget the seriousness with which he took his role protecting MIT and those of us who consider it home.”
During the pursuit, a MBTA transit police officer was seriously injured and transported to the hospital, the news release states. He was identified as Richard H. Donahue Jr., 33, and was being treated at Mt. Auburn Hospital..
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-Eleven in Cambridge, just across the Charles River from Boston.
Dozens of officers and National Guard members descended on Watertown shortly after the shooting outside a building on MIT's campus in Cambridge, according to the Associated Press.
Authorities were calling for somebody to get on the ground and put their hands up and a loud thud was heard after someone shouted "fire in the hole," the news agency reported.
Witnesses told The Associated Press they heard multiple gunshots and explosions at about 1 a.m. Friday. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.
State police spokesman David Procopio told news agency, "The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers."
Doctors at Beth Israel Deaconess Medical Center in Boston where a suspect in the marathon bombings was taken and later died said they treated a man with a possible blast injury and multiple gunshot wounds. They wouldn't say if the patient they treated, who came in with police, was the suspect in the black hat from marathon surveillance footage.
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.
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.
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.
Fox News' Jennifer Griffin, Jana Winter, Mike Tobin, Mike Levine, Griff Jenkins and The Associated Press contributed to this report.
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