The suspect at large is 19-year-old Dzhokhar Tsarnaev and was born in Kyrgyzstan, NBC News said. He had a Massachusetts driver's license, it said.
The dead suspect was identified as 26-year-old Tamerlan Tsarnaev. He was born in Russia.
Police and law enforcement agents launched a massive manhunt to find the younger of the two suspects, ordering a complete and unprecedented lockdown of the entire city.
The dramatic developments unfolded just hours after the FBI released pictures and videos that showed the two suspects walking with backpacks on the sidewalk in the direction of the finish line of the Boston Marathon.
FBI said one of the videos showed the younger suspect placing his backpack at the site of the second explosion.
NBC quoted law enforcement officials as saying that Tamerlan Tsarnaev was run over by a vehicle during a firefight with police officials near the MIT campus late last night.
They also told NBC News that the brothers entered the US with family in 2002 or 2003, and that Tamerlan Tsarnaev became a legal permanent resident in 2007.