Authorities say a 15-year-old boy has been arrested after carrying an unloaded assault pistol onto a Los Angeles County commuter train. The Antelope Valley youth was arrested Tuesday after the Metrolink train pulled into the Santa Clarita station, north of Los Angeles.

Authorities say the youth, whose name was withheld because of his age, had a Mac-10 or Mac-11 pistol in his backpack and had bragged about it to a passenger, who told the conductor reports the Associated Press.

Authorities say the weapon had been converted to fire on full automatic but was unloaded. No ammunition was found, and sheriff’s officials say they don’t know what the teenager was planning to do with the gun.

The boy was booked at Sylmar Juvenile Hall and could face charges of illegal weapons possession.