A 14-year-old boy has been charged with murder in the fatal stabbing of a 13-year-old on a city bus on Friday afternoon on Staten Island.

The suspect, whom the police have not named, was also charged with manslaughter and criminal possession of a weapon. He remained in custody on Saturday, the police said; it was unclear whether he had a lawyer.

At a news conference on Friday, John M. Chell, the New York Police Department’s chief of patrol, called the killing a “tragic, tragic homicide” and said that the police believed it was gang-related.

Just before 2:30 p.m. on Friday near Littlefield Avenue on Hylan Boulevard, one of the borough’s major thoroughfares, an argument broke out on the bus between the victim, whom the police have identified as Syles Ular, and the suspect, Chief Chell said. A knife was seen and gang signs were flashed back and forth, he added.

Syles was stabbed multiple times and taken to Staten Island University Hospital South, where he was pronounced dead, the police said.

The 14-year-old fled the scene and ran about three blocks, Chief Chell said, before he was caught and held by a retired police sergeant who happened to be in the area and had pursued him after a bystander said he had stabbed someone.

Police officials at the news conference mentioned two other incidents on Friday involving weapons and teenagers. Around noon, Chief Chell said, a 16-year-old girl in Brooklyn was shot in the ankle as she walked to a bus stop. Later that afternoon, students at two adjacent high schools in Manhattan were placed on “hard lockdown” after a report that a 15-year-old had brought a gun to school. No one was injured, but the police later recovered two guns and an “imitation pistol” from three different students, Chief Chell said.

A few hours later, at about 9:18 p.m. on Friday, a 17-year-old girl was fatally shot in the head in an apartment on Staten Island, the police said.