Airplanes were barred from taking off Monday night from Chicago's Midway Airport (search) after an undisclosed security problem.

About 20 planes were kept on the ground and passengers in the terminals were told to leave, said Chicago Aviation Department (search) spokeswoman Annette Martinez. She said passengers on about 20 other planes weren't allowed to disembark.

Martinez declined to provide details about the incident, which happened about 7 p.m. at a Transportation Security Administration checkpoint.

Passengers were to be screened again before being allowed to return to the gates. About 500 passengers were affected by the delays, she said.