BOSTON UPDATES

By Pete Williams, Richard Esposito, Michael Isikoff and Tracy Connor, NBC News
Boston and its surburbs, universities and transit system were on total lockdown Friday as police hunted for marathon bombing suspect Dzhokhar Tsarnaev — on the loose after his accomplice brother was killed in a stunning chain of events that left one cop dead and another injured, officials said.
Authorities were confronting a double-edged nightmare: a ruthless killer at large in a densely populated area and a four-mile stretch of road possibly littered with explosive devices tossed from the suspects’ getaway vehicle during a wild chase and firefights. An explosive was found in Boston and disabled, an official said.
A possible associate of the brothers was also being sought.
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Two unidentified people were taken into custody at the Cambridge, Mass., home where Dzhokhar Tsarnaev and his older brother Tamerlan grew up, but they were not being described as additional suspects. Three dozen FBI agents were surrounding the house.

NBC’s Lester Holt reports from Watertown, Mass., where Boston and state police, SWAT teams and a helicopter are engaged in a tense search for a suspect in the Boston Marathon bombing who is still at large.
Across the area, as police cars screamed down streets and helicopters hovered overhead, authorities urged the public to stay inside, their doors locked to anyone but law-enforcement officers.
“There is a massive manhunt under way,” Massachusetts Gov. Deval Patrick said. “We are asking people to shelter in place.”
The lockdown initially affected more than 300,000 people in Cambridge, Watertown, Newton, Brighton, Allston and Belmont, but by 8 a.m., the entire city of Boston was paralyzed, officials said.
Watertown, where the second suspect was last seen, was the epicenter of the search. Frightened residents were trapped in their homes as convoys of heavily armed officers and troops arrived by the hour.
Harvard University, Boston University, the Massachusetts Institute of Technology and Emerson University were all closed and students were told to stay inside. Boston public schools were shuttered for the day.
The overnight violence began near MIT about five hours after the FBI released surveillance photos of the two men suspected of planting two bombs near the finish line of Monday’s Boston Marathon, killing three and wounding 176.
Read more: Who are the brothers accused of the Boston Marathon bombing?
Tips about the identity of the suspects were still pouring in when the Tsarnaev brothers robbed a 7-Eleven then fatally shot MIT police officer Sean Collier, 26, in his vehicle at 10:20 p.m., law enforcement officials said.
The brothers — of Chechen origin, but legal permanent residents of the U.S. who moved here a decade ago — then carjacked a Mercedes SUV, holding the driver captive for a half-hour before letting him go at a gas station in Cambridge, sources said.
The suspects took off in the car toward Watertown, tossing explosive devices out the window, officials said.

Uncredited / AP
FBI photos of the Boston Marathon bombing suspects, brothers from the Chechen Republic. The man on the left is dead. The man on the right, Dzhokhar Tsarnaev, is still at large.

There was a long exchange of gunfire, Andrew Kitzenberg of Watertown, who took photos of the clash from his window and spread them on social media, told NBC News in an interview.
“They were also utilizing bombs, which sounded and looked like grenades, while engaging in the gunfight,” he said. “They also had what looked like a pressure-cooker bomb.
“I saw them light this bomb. They threw it towards the officers,” he said. “There was smoke that covered our entire street.”
A transit officer, identified as Richard H. Donahue, 33, was seriously injured during the pursuit. Authorities said he was in surgery at Mount Auburn Hospital.
Kitzenberg said he saw the firefight end when Tamerlan Tsarnaev ran toward the officers and ultimately fell to the ground.
Tamerlan— the man in the black hat from FBI photos released six hours earlier — had an improvised explosive device strapped to his chest, law enforcement officials said.

Dzhokhar — who was wearing a white hat in the surveillance photos from the marathon — drove the SUV through a line of police officers at the end of the street, Kitzenberg said.
Police said Dzhokhar has a Massachusetts driver’s license and lived in Cambridge. He was described as light-skinned and with brown, curly hair, and wearing a gray hoodie. The FBI was releasing more photos of him.
“We believe this man to be a terrorist,” Boston Police Commissioner Ed Davis said. “We believe this to be a man who’s come here to kill people.”
Armored humvees and busloads of law-enforcement could be seen rolling into Watertown in the hours after the gunfight. A photo showed two officers in military gear lying on a backyard shed with their weapons trained on a home.
“We’ve got every asset we could possibly muster on the ground right now,” Patrick said.
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Attorney General Eric Holder and FBI Director Robert Mueller headed to the White House to brief President Obama on the developments.
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