A 15-year-old boy was in police custody after allegedly shooting a 17-year-old boy on a school bus Friday morning, police said.

Police told The Philadelphia Inquirer the unidentified suspect allegedly used a .25-caliber handgun to shoot a fellow student on the way to their West Philadelphia high school.

The unidentified victim, who suffered a bullet graze to his head, was expected to survive.

Police said it was unclear what prompted the shooting in the back of the bus, while Police Deputy Commissioner Kevin Bethel criticized how easy it is for teens to obtain firearms.

"The ease of these kids to get guns is something that troubles us on a daily basis," Bethel said.

Police said following the shooting, the teen suspect allegedly used his weapon to threaten the bus driver and fled the vehicle once it was stopped.

The suspect was soon caught by police officers nine blocks away.