MUELHEIM, West Germany (AP) _ A commuter airliner was struck by lightning and crashed Monday, killing all 21 people aboard, authorities said.

Witnesses said a wing fell off just before the craft crashed in a rural area near Muelheim, about seven miles north of its destination, the Duesseldorf airport.

The plane had been bombarded by lightning and hail and probably began to disintegrate before it crashed, said Manfred Kueppers, a spokesman for federal aviation investigators.

''There was a fireball in the air,'' a police official said, describing witnesses' reports.

Police said victims' bodies were burned or mutilated beyond recognition, and that wreckage of the twin-engine turboprop FA-4 Metroliner was scattered across a wide area.

One witness, Helmut Mrosek of nearby Essen, said the sound of a lightning bolt striking the aircraft ''was as if a giant pounded on a sheet of steel with a large hammer.''

''There was a bolt of lightning, and then I saw an airplane plunge out of the clouds toward our house. But at the same moment the pilot was able to right the aircraft,'' said another witness, who asked not to be identified.

But immediately after that, the aircraft ''fell like a rock,'' the witness said.

Snow and rain squalls with gusty winds were moving through the region at the time of the crash, officials said.

Hans-Ulrich Ohl, a spokesman for federal flight control authorities in Frankfurt, said the plane crashed at 7:57 a.m., 41 minutes after takeoff from Hanover.

The plane belonged to the Nuremberg Flight Service, and 19 passengers and a crew of two were on board at the time of the crash, the spokesman said.

''The aircraft just disappeared from our radar screens without warning,'' the spokesman said.

A Nuremberg Flight Service official said the flight crew had not reported any difficulties before the plane went down. There had been no previous crashes of an FA-4 Metroliner in West Germany, said the official, who spoke on condition of anonymity.

Monday's plane crash was the worst in West Germany since 1971, when 21 people died in a crash near Hamburg.