A gun goes off on a CTA bus, wounding a passenger.Police are now trying to find out who left the loaded weapon on board.

A woman was grazed in the leg while riding a CTA bus this morning when another passenger picked up an "unattended gun" from a seat and it went off, officials said.

A man sitting across from the woman noticed the gun, only about 3 inches long, and picked it up thinking it was a toy or a lighter and it discharged, police on the scene said.

The .22-caliber bullet struck a vent underneath a row seats and a fragment hit the 47-year-old woman around 6:35 a.m., police and the agency said. The fragment did not penetrate her clothes but she was taken to a hospital for observation.

The woman told WGN-TV she was on her way to work when she was struck by the bullet.  She said there were not a lot of people on the bus at the time.

The No. 47 bus was heading east on 47th Street when the incident occurred at Kildare, according to CTA spokeswoman Catherine Hosinski.  Passengers were transferred to the next bus on the route.