ST. LOUIS (AP) _ The left engine on an American Airlines jet caught fire Friday during a flight to Chicago, forcing the pilots to return to St. Louis and evacuate the aircraft on the runway.

There were no reports of injuries aboard the MD-80 jet, said Jeff Lea, spokesman for Lambert-St. Louis International Airport. All 137 passengers, two pilots, and three flight attendants left the plane safely after it turned back to Lambert and landed, said Tim Wagner, spokesman for Fort Worth, Texas-based American Airlines.

Flight 1400 reported engine problems sometime after takeoff and landed at 1:40 p.m. CDT. Passengers walked down a staircase to the runway after emergency crews extinguished the fire with flame-retardant foam.

American Airlines maintenance will look at the aircraft and try to determine what cased the fire.

The National Transportation Safety Board may get involved, he said.

The NTSB did not immediately return a phone call seeking comment.

Cari Slote, a medical coding consultant from Jefferson City, said she was on a different flight arriving in St. Louis when the pilot made an announcement.

"He said we couldn't get to our gate because American had an emergency aircraft coming in," she said.

When her plane landed, it seemed fairly calm, she said, although she did spot a fire truck on the runway.