PERRIS, Calif. (AP) _ Contaminated fuel was a factor in the killer crash of a skydiving plane and the pilot had no formal training in the craft, federal investigators said.

Contaminants found by National Transportation Safety Board investigators caused the right engine to fail, said Don Llorente, the agency's supervisor in Los Angeles. Engines ''just don't run on that substance,'' he said.

The DeHavilland DHC-6 Twin Otter crashed Wednesday, killing 16 of 22 people on board. The pilot and co-pilot were among the dead.

Investigators traced the contamination to a fuel delivery made the day before the crash, Llorente said in Saturday's Press-Enterprise of Riverside.

The airport ordered fuel on Saturday from its regular supplier. The delivery was made Tuesday, but the airport's fuel reservoir pump broke down, investigators said. The airport asked the supplier to help transfer the fuel from the reservoir to a truck that was used to refuel planes.

Ben Conatser, who owns the airport and the plane, said the equipment used to transfer fuel had been used to pump other liquids and had not been ''purged'' to remove cleaners and solvents.

In addition, the NTSB found that the pilot and copilot had no formal training in flying the type of craft that crashed, Conatser said.

The practice of a qualified pilot informally training other pilots who don't have formal instruction is not forbidden by the Federal Aviation Administration.

Six survivors remained hospitalized Friday. Most were improving but one slipped to critical condition.

The NTSB said Thursday that none of the 20 parachutists on board the plane wore required safety belts and the pilot and copilot did not wear shoulder harnesses, which may have contributed to the high death toll.

However, NTSB officials stopped short of saying that lives could have been saved had the restraints been worn. The pilot and co-pilot wore waist belts but were not required to wear shoulder harnesses.

The airport is an internationally known center for skydiving in rural Riverside County, 70 miles southeast of Los Angeles.