*** UPDATE ***
NBC News is reporting that the two suspects are brothers age 19 and 20. NBC News further reports that the suspect that is still at-large is named Dzhokar Tsarnaez.
Stay with NBC25 and the Today Show for more on this story.
*** END UPDATE ***
Authorities say one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is underway for another.
Residents of the Boston suburb of Watertown have been advised to keep their doors locked and not let anyone in.
The Middlesex district attorney says the two men are suspected of killing an MIT police officer at the college late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed. Hours earlier, police had released photos of the bombing suspects and asked for the public's help finding them.
Authorities say the suspects threw explosives from the car as police followed it into Watertown. The suspects and police exchanged gunfire and one of the suspects was critically injured and later died.
Police say the suspect on loose is a "terrorist" who "came here to kill people."