A 39-year-old father of two whom witnesses described as an innocent bystander was shot and killed by a teenage gunman on an MTA bus in Brooklyn Thursday evening, authorities say.

Law enforcement sources say the 14-year-old boy was sitting in the back of the B15 bus when other kids got on and the fight erupted; he pulled out a revolver and fired several shots, one that hit Angel Rojas in the head.

The boy was taken into custody and a gun was recovered at the scene, near Lafayette Avenue and Marcus Garvey Boulevard in Bedford-Stuyvesant. The teenager was charged Friday with second-degree murder, among other offenses.

Witnesses said Rojas, who was heading from one job to another, was an innocent bystander who was caught in the middle of an altercation that broke out among a group of young men on the bus.

"They sounded like Roman candles, like fireworks. One shot, then four repeated shots after that," said Tiger Frazier, a resident in the neighborhood who recorded some of the chaotic aftermath on a cellphone.

The Bedford-Stuyvesant Volunteer Ambulance Corps happened to be across the street from where the bus stopped, and three instructors and three cadets who were in the middle of training jumped into action.

"It was absolute hysteria," said instructor Shanida Robinson.

Rojas, of Brooklyn, was taken to Woodhull Hospital in critical condition, where he died, police said.

His devastated widow and children were in disbelief at their home Friday.

His 12-year-old son, Saury, said Rojas "was a nice dad" who worked in two different grocery stores to support his kids and would still make time for family outings on Sundays.

"Why did it have to be him on the bus," Saury said.

Rojas' widow, Maria Lopez, said in Spanish, with Saury interpreting, that she couldn't believe she had lost her husband of 14 years.

She said she doesn't know what she and her children are going to do now.

Lopez said she wants the teen shooter held accountable so that no one else gets hurt.

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