"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."
Around midday, the suspects' uncle Ruslan Tsarni of Montgomery Village, Md., pleaded on television: "Dzhokhar, if you are alive, turn yourself in and ask for forgiveness."
Authorities said the man dubbed Suspect No. 1 — the one in sunglasses and a dark baseball cap in the surveillance-camera pictures — was Tamerlan Tsarnaev, while Suspect No. 2, the one in a white baseball cap worn backward, was his younger brother.
Exactly how the long night of crime began was unclear. But police said the brothers carjacked a man in a Mercedes-Benz in Cambridge, just across the Charles River from Boston, then released him unharmed at a gas station.
They also shot to death a Massachusetts Institute of Technology police officer, 26-year-old Sean Collier, while he was responding to a report of a disturbance, investigators said.
The search for the Mercedes led to a chase that ended in Watertown, where authorities said the suspects threw explosive devices from the car and exchanged gunfire with police. A transit police officer, 33-year-old Richard Donohue, was shot and critically wounded, authorities said.
Some 200 spent shells were found afterward.
Dzhokhar Tsarnaev somehow slipped away. He ran over his already wounded brother as he fled, according to two law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation. At some point, he abandoned his car and ran away.
Tamerlan Tsarnaev died at a Boston hospital after suffering what doctors said were multiple gunshot wounds and a possible blast injury.
The brothers had built an arsenal of pipe bombs, grenades and improvised explosive devices and used some of the weapons in trying to make their getaway, said Rep. Dutch Ruppersberger, D-Md., a member of the House Intelligence Committee.