Crewmembers on United flight No. 972 alerted the airport's Air Traffic Control Tower of the bag at about 10:40 a.m., about 30 minutes after the plane was scheduled to depart for Chicago, airport spokesman Mike McCarron said.
The plane was relocated to a secure, remote area and the passengers and crew were removed while San Francisco Fire Department hazardous materials and explosive ordinance teams investigated the plane.
The owners of the bag did not speak English well enough to explain what was in the bag, and after interviewing three passengers -- including two who apparently owned the bag -- and searching the aircraft, investigators determined the plane was safe, McCarron said.
Passengers were allowed back onboard the plane at about 2 p.m., and the aircraft was scheduled to depart at 3:45 p.m.
No one was arrested or detained as a result of the incident.