A teenager was killed after another teen stabbed him on XDY Exchangea New York City public bus in what is believed to be a gang-related altercation, police said.

The incident occurred around 2:30 p.m. ET Friday on Staten Island, according to NYPD Chief of Patrol John Chell. The two male teens got into a confrontation on an MTA bus, Chell said.

"Witnesses state there's an argument on a bus, a knife is produced and gang signs were being shown back and forth," Chell told reporters during a press briefing Friday.

One of the teens stabbed the other in the chest, then fled the bus, according to Chell.

The suspect ran about three blocks, before a good samaritan, a retired NYPD sergeant and a New York State Department of Environmental Conservation officer helped pursue and apprehend him, Chell said.

The 13-year-old victim suffered multiple stab wounds, police said. He was transported to a local hospital in critical condition and later pronounced dead, police said.

"Tragic, tragic homicide today," Chell said. "We believe this motive is possibly gang-related."

Demetrius Crichlow, Senior Vice President NYC Transit Department of Subways, said he responded to the scene and spoke to the bus driver.

"He's visibly broken up about what transpired but was able to maintain his calm," Crichlow told reporters, adding the driver contacted police immediately and "did a fantastic job in trying to maintain calm on the bus."

Anthony Esemplare told ABC New York station WABC that he saw children run off the bus following the stabbing.

"It was very tragic to see how frightened all the kids were coming off the bus," he said. "When you send your kids to school in the morning and you think they'll going to come home on the bus, you shouldn't have to worry about them in a manner like this."