The investigation of the crash of an American Airlines DC10 is focusing on three support mechanisms that joined the jet's engine assembly with its left wing.

The three key parts are a 3 1/2-inch bolt on the engine assembly and two superstrength metal attachments - one forward and one rear - that hold the engine assembly to the wing.

Failures in two of these pieces are thought to have caused the engine to fall off: a shearing in the bolt and a fracture in the rear attachment.

Investigators theorize that the shearing in the bolt shifted excessive force to the front and rear attachments. The front attachment, which is larger and stronger, held. The rear attachment gave way, possibly due to vibrations caused after the bolt split.

"Somewhere along the line, that was the final failure that allowed the engine to drop down," said John D. Rawson, an air safety investigator for the Federal Aviation Administration.

When the rear attachment broke away, officials theorize, the 11,000-pound General Electric engine pivoted up and over the wings, snapped off the front attachment, cartwheeled toward the rear of the aircraft over the tail assembly and finally crashed to the ground.

Investigators believe that the bolt was "the key that set up the sequence of events," said James M. Vines, acting director of the Federal Aviation Administration's flight standards service. "The only part we have that we know about is the bolt.We know it gave way, but the sequence has not been fully established."

The theory was drawn up after investigators at the scene of the crash discovered the severed bolt and found that the rear attachment was ripped off the engine assembly and the front attachment was broken.

The bolt that sheared is part of a mechanism designed to allow the wing and the engine assembly to withstand the thrust of flight - about 40,000 pounds at maximum power. The front and rear attachments are designed to take sideways and up and down loads, not forward thrust.

FAA officials said yesterday that bolts can be tested for strength in a variety of ways.

For example the bolt can be removed from the aircraft, magnetized and then covered with a paint that contains magnetized particles, accident investigator Rawson said. A special light is then focused on the bolt, causing the particles to align themselves in any existing crack, Rawson said.

The testing procedure, which requires removing a sheet metal plate from the engine assembly withdrawing the bolts, takes two to three hours, FAA officials said. After Faa Administrator Langhorne Bond ordered special testing yesterday, most air carriers elected simply to replace the bolts altogether, rather than go through time-consuming testing process, officials said.