Investigators have determined that metal fatigue induced by an electrical current of unknown origin caused the engine on the Northwest Airlines DC10 jetline to break up over Leesburg last Jan. 31, according to knowledgeable sources.

As a result of the problem, the huge engine covering, called a cowling, fell from the airplane and landed in a Leesburg back yard. Shrapnel-like pieces of metal from the giant fans in the disabled engine penetrated another engine on the jet, but did no serious damage. The plane returned safely to Dulles International Airport, where it had just taken off.

One of the 46 blades on the front fan of the plane broke off, investigators think. That put the entire fan badly out of balance and it began to disintegrate. A crack caused by metal fatigue has been discovered in the first fan blade and is regarded as the origin of the incident.

Fatigue can occur in metal when it is stressed too often, as happens when a paper clip is bent back and forth until it snaps in two. In this instance, a crack was induced into the fan at some point during its history by an electrical arc. Under the stress of normal operations, the crack grew until the blade broke in two.

Federal officials regard the incident as unique, and not just one that signals a potential problem with the Pratt and Whitney JT9D engine, which powers Northwest's DC10s and many Boeing 747s and Airbus Industrie A300s. All are jumbo jets.

Nonetheless, Pratt and Whitney sent a message Feb. 10 to the users of the JT9D, advising them that the break in the fan was the result of metal fatigue initiated by "an arc burn. . . . You are reminded to use caution to avoid contact with any electrical source. . . ."

There could be several potential sources of electrical arcing around engine maintenance shops. Electric currents are used for some types of inspections and the fan blades, made of titanium, must be examined for possible cracks about every three months. Other theoretical possibilities are electrical welders for work done around engines, a live electrical wire in the area, or even a lightning strike.

The National Transportation Safety Board is continuing its investigation of the incident and has reached no conclusions.

In a related development this week, the last missing piece from the engine was found in a creek by Leesburg Sunday by 17-year-old Charles Arthur, the son of American Airlines pilot Claude Arthur. Young Arthur, his mother Caroline said, came home from walking the dog and announced he had found part of an airplane.

"I said, 'Oh yeah,'" she said. "The next day he told his father and they went out and looked for it. Sure enough it was. His father called Northwest Airlines."

The part is a 95-inch-diameter ring that is supposed to hold the cowling to the engine. It has been sent to Pratt and Whitney for examination.