The incident was reported on a northbound B39 bus at White Plains Road and Van Nest Avenue around 4:30 p.m.
Police say an 18-year-old was riding the bus when he was approached by the suspect, who started interrogating him.
The victim was nervous and sent a text to his 36-year-old uncle and asked him to meet him at the bus stop for help.
The suspect apparently became angry with the uncle and a physical struggle broke out.
Authorities say the suspect displayed a firearm and discharged one round, which stuck the bus accordion's ceiling.
The suspect ran away from the scene, northbound on White Plains Road.
No injuries were reported and no arrests have been made.
The investigation is ongoing.
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