CHICAGO (AP) _ A passenger ran past a security checkpoint and disappeared into the crowd at O'Hare International Airport on Thursday, prompting United Airlines to shut down a terminal for almost three hours as authorities searched for him.

United canceled 81 flights and expected delays of several hours on others, spokesman Joe Hopkins said.

The man was not found and Chicago Police said officers turned up nothing suspicious.

About 6,000 passengers were cleared from the terminal during the search, airline officials said.

The man was leaving a concourse and heading toward the baggage claim area when he turned back and ran up a one-way staircase leading to the concourse, said United spokesman Matthew Triaca. Guards are stationed at the staircase, but the man ran past them.

No United flights left the terminal during the search and only a few landed at other United gates. United had resumed flights by early evening.