Preliminary reports indicated there was not an attempted hijacking, said FAA spokeswoman Marcia Adams. No one was injured, she said. The jetliner landed without incident at Chicago's O'Hare International Airport.

A young adult, described later by his father as having a history of mental illness, was subdued aboard Flight 1238 from Los Angeles to Chicago after trying to get into the cockpit, said FBI spokesman Ross Rice.

"The male individual was physically restrained by other passengers," American spokesman Al Becker said. "This appears to be an isolated incident."

The captain of the Boeing 767, which was carrying a crew of nine and 153 passengers, declared an emergency. The F-16s were then dispatched to escort the plane, Chicago Aviation Department spokeswoman Monique Bond.