BOSTON -- The largest city in New England was almost entirely locked down on Friday as authorities conducted an intensive manhunt for the remaining suspect wanted in connection to the bombing attacks on the Boston Marathon this week.
The FBI and local authorities searched for 19-year-old Dzhokhar Tsarnaev on Friday, after an overnight police chase and gun battle led to the death of his brother, Tamerlan -- the other suspect in the bombing.
A few hours after the FBI released their photos, officials said the suspects shot and killed a 26-year-old security officer at the Massachusetts Institute of Technology as he sat in his patrol car.
Authorities said the brothers then made a desperate attempt to flee by carjacking a Mercedes Benz SUV, telling the driver that they were the marathon bombers. Police said the brothers held the driver captive for a half hour, trying to use his bank card to get cash from ATMs.
The driver was ultimately released, unharmed, at a gas station in Cambridge, Mass.
Authorities located the suspects and began chasing them as the stolen SUV sped toward Watertown, Mass. At one point, police say, the suspects tossed explosive devices out the window. A barrage of gunfire followed.
"They were utilizing bombs, which sounded and looked like grenades, while engaging in the gunfight," witness Andrew Kitzenberg told NBC News. "I saw them light this bomb. They threw it towards the officers... there was smoke that covered our entire street."
Kitzenberg said the gun battle ended when Tamerlan Tsarnaev lunged toward the police officers and suddenly fell to the ground. Authorities later said he had an explosive device strapped to his chest. However, it did not detonate. Police said Dzhokhar escaped in the stolen SUV. Authorities have been searching for him ever since.
A transit officer was injured during the police chase and was taken to the hospital in serious condition, officials said.
Boston's subway and bus transit system was shut down during the manhunt. Amtrak service to Boston was suspended. Harvard, MIT, Boston University and Emerson University were all closed Friday.
The lockdown initially affected more than 300,000 people in the suburb towns of Cambridge, Watertown, Newton, Brighton, Allston and Belmont. But by 8 a.m., the entire city of Boston was paralyzed, officials said. Authorities warned people to stay inside their homes and lock their doors and windows.
Authorities said the suspects emigrated to the United States from Chechnya a decade ago, seeking asylum. A motive for the bombings is still unknown.
The brothers' uncle -- who called the suspects "losers" -- made a public plea from his Maryland home for the remaining suspect to turn himself in to authorities.
"We are ashamed," he said.
The boy's father in Russia told the Associated Press that Dzhokhar Tsarnaev is a "true angel." Police, however, paint a starkly different picture of the man.
"We believe this man to be a terrorist," said Boston Police Commissioner Ed Davis. "We believe this to be a man who's come here to kill people."