Investigators don't yet know why the tail fin and rudder broke off in flight just before American Airlines 587 crashed, but such a catastrophic loss has occurred just once before in commercial aviation history.

On Aug. 12, 1985, a Japan Air Lines jumbo jet lost its vertical tail section on a flight from Tokyo to Osaka. The Boeing 747 flew in circles for half an hour before crashing into a 7,000-foot mountain, its pilots still trying desperately to understand why they had lost control.

That crash killed 520 people, the worst single-aircraft mishap in commercial aviation. Four people survived.

In Monday's crash, the American Airlines Airbus A300 took off from Kennedy International Airport and shortly afterward lost its vertical stabilizer and rudder. Without this two-part tail assembly, the jetliner would have suffered a loss of stability and turning control.

The plane plunged into a Queens neighborhood, killing all 260 people on board and five people on the ground.

Investigators said witnesses described a "wobble," and the cockpit voice recorder revealed "suggestions of a loss of control" 17 seconds before the plane crashed.

The 27-foot tail fin and the rudder have been pulled out of Jamaica Bay and taken to a nearby collection center for study. Both appeared intact with little or no visible damage.

Most forces exerted on an aircraft are from front to rear. The tail fin is made of aluminum or composite material, and is designed to flex from side to side, but whether it could be snapped off by a lateral force was unclear.

National Transportation Safety Board experts said they did not know why the tail section was sheared cleanly away from the fuselage.

"We'll be looking very carefully at how the tail failed," NTSB investigator George Black Jr. said Wednesday.

Besides the commercial accidents, the only other recorded cases of tail fin losses involved an Air Force B-52 bomber, a Boeing E8 and a Convair 880 jetliner, all during test flights decades ago, according to Scott Haskin, an aircraft maintenance specialist and industry historian.

In the 1985 Japan Air Lines crash, the aircraft suffered "massive decompression" — a sudden loss of cabin pressure — when the dome-shaped pressure seal in the rear of the passenger compartment unexpectedly collapsed.

The explosive force destroyed the aircraft's hydraulic lines that converged in the tail, and ripped away the vertical stabilizer and rudder.

Unable to see the plane's rear, the cockpit crew did not know they had lost the tail, only that the aircraft's control surfaces — flaps, elevators and rudder — were suddenly and mysteriously inoperative.

Capt. Masami Takahama told air controllers that a rear door had broken, declared an emergency and was cleared to land at either of two nearby airports.

Takahama was able to steer the crippled plane by applying and easing power to the engines, but with no rudder to control the turns, the jetliner turned in circles, unable to set a course for either runway.

Photographs taken by witnesses on the ground clearly showed the plane's tail fin was missing.

Investigators eventually found that the plane's rear pressure dome, damaged earlier in a "hard landing," had been improperly repaired, and eventually gave way during the Tokyo-Osaka flight. Boeing, which had supervised the pressure dome repairs, took responsibility for the failed repairs.