By Pete Williams, Richard Esposito, Michael Isikoff and Tracy Connor, NBC News
Boston and its suburbs, 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.
During a desperate effort to flee after their photos were released by the FBI, the brothers carjacked a Mercedes SUV and told the driver they were the men behind Monday's double-blast attack at the race and had just killed a campus security officer, a source told NBC News. The driver was released unhurt.
The older brother, Tamerlan Tsarnaev, was killed after a wild chase and firefight, but Dzhokhar was still at large in a densely populated area, and an associate was being sought. Adding to the nightmare, an explosive was found in Boston Friday morning and disabled, an official said.
The suspects' uncle, Ruslan Tsarni, called the brothers "losers" and urged Dzhokhar to turn himself in. "We're ashamed," he thundered outside his Maryland home.Three dozen FBI agents were surrounding the Cambridge, Mass., home where the brothers, of Chechen origin, grew up after moving to the U.S. a decade ago, seeking asylum. They feared it could be booby-trapped, Boston police sources said.
Across the area, as police cars screamed down streets and helicopters hovered overhead, authorities urged the public to stay inside and keep 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."
Subways and buses were shut down, and Amtrak service to Boston was cut. Harvard University, Boston University, the Massachusetts Institute of Technology and Emerson University were all closed and students were told to stay inside. The University of Massachusetts' Dartmouth campus was being evacuated because someone wanted in the case is registered at the school.
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 a sprawling search. Frightened residents were trapped in their homes as convoys of heavily armed officers and troops arrived by the hour.
The overnight violence began at 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 fatally shotMIT patrol officer Sean Collier, 26, in his vehicle at 10:20 p.m., law enforcement officials said.
The brothers then carjacked a Mercedes SUV, holding the driver captive for a half-hour while they tried to use his cash card to get money from three ATM's, a source said. At the first, they put in the wrong number; at the second, they took out $800 and at the third, they were told they had exceeded the withdrawal limit, the source said.
The man was released unharmed at a gas station in Cambridge, sources said. As they sped toward Watertown, a police chase ensued and the suspects tossed explosive devices out the window, officials said.
There was a long exchange of gunfire, according to Andrew Kitzenberg of Watertown, who took photos of the clash from his window and spread them on social media.
“They were also utilizing bombs, which sounded and looked like grenades, while engaging in the gunfight,” he told NBC News in an interview. “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 lives in Cambridge. He was described as light-skinned and with brown, curly hair, and wearing a gray hoodie. The FBI released more photos of him, including a surveillance camera photo from a 7-Eleven.
His father, in Russia, told the Associated Press he was "a true angel" and described him as a medical student who was expected to visit for the holidays.
Authorities painted a starkly different picture.
"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.
"We've got every asset we could possibly muster on the ground right now," Patrick said.
Later Friday morning, attorney General Eric Holder and FBI Director Robert Mueller headed to the White House to brief President Obama on the developments.
Secretary of State John Kerry, who is from Massachusetts, praised law enforcement for their work during "a pretty direct confrontation with evil."
"In the past few days we have seen the best and we've seen the worst of human behavior, and it's the best that all of us really want to focus on," he said.
Senate Intelligence Committee chairwoman Dianne Feinstein told NBC News it's clear the marathon bombing was "a terrorist attack."
"It's also clear, you know, that you know you don't have to be a card-carrying member of any terrorist group to commit a terrorist attack," she said.
NBC News' Kasie Hunt contributed to this story