According to METRO police, the woman and her boyfriend were among the passengers on a regular bus route 29 along Cullen Boulevard. The shooting happened as the bus crossed under the 610 South Loop, officials said.
Investigators initially thought it was a domestic dispute; however, after reviewing METRO's onboard surveillance video, investigators said the shooting appears to be accidental.
METRO said the video shows the man talking and fumbling with his gun, which then unintentionally discharged, hitting his girlfriend in the leg. METRO said it does not appear he was trying to shoot anyone, though the investigation is still in its early stages.
The man did not stay at the scene, according to investigators. METRO police are now looking to identify him.
The woman was taken to a local hospital with non-life-threatening injuries, officials said.