An anonymous bomb threat prompted a Southwest Airlines (search) flight to return to Midway Airport and land shortly after it took off Friday afternoon, officials said.

Chicago Department of Aviation spokeswoman Monique Bond said that after Flight 1003 bound for Phoenix took off at 3:05 p.m., an anonymous caller phoned the Chicago Police Department's 911 center and said there was a bomb aboard.

No bomb has been found, Bond said.

"The threat was specific enough in nature that the decision was made to bring the plane back to Chicago," Southwest spokesman Ed Stewart said.

The jet returned to Midway and landed at about 3:40 p.m. The 123 passengers and five crew members left the plane and the aircraft was in a secure location away from the terminal, Bond said.

She said law enforcement agencies, including the FBI (search) and the Chicago Police Department (search), were searching the plane.