An embarrassing incident at an American Airlines maintenance base in Fort Worth left a plane badly damaged, after the landing gear retracted, sending the nose of the jumbo jet crashing to the ground.

The Boeing 767-300 was undergoing a heavy maintenance check at the company's Alliance Airport hangar, when the nose gear suddenly folded up, as it would if the plane was in the air.

A source provided photos of the plane to NBCDFW.com.

At the time, mechanics were running the aircraft through a series of functional tests, according to Tim Wagner, a spokesman for the airline.

Amazingly, no workers were injured when the gear collapsed. During ground tests, mechanics often sit in the cockpit or stand near the belly of the plane.

The plane has been out of service since the July 15 incident. The airline was trying to determine the extent of the damage, Wagner said Thursday.

The company plans to conduct an internal investigation to determine how the accident happened, he said.

An airline source who asked not to be identified because of the sensitivity of the situation said that the damage may be severe.

According to Boeing, the 2008 list price on a 767-300 was between $144 to $161 million.