The plane that crashed Sept. 5 in western San Juan County encountered "instrument flying conditions" over western Colorado, but neither of the two pilots aboard were qualified to fly in such bad weather.

That's one of the conclusions in a preliminary report released Thursday by the National Traffic Safety Board, which is investigating the plane crash that killed four California residents.

The NTSB report indicates the crash occurred about 2:08 p.m. Sept. 5, when the twin-engine Cessna 310H "impacted mountainous terrain at an elevation of about 11,500 feet."

The wreckage was located the next day near the head of Cascade Creek at Grizzly Peak, about nine miles west of Silverton.

Weather was partly cloudy in Silverton that day, but there were some rain squalls and a thunderstorm was reported in the area.

The preliminary NTSB report also says that the two pilots aboard were not rated to fly a twin-engine plane and were not operating on a flight plan.

Additionally, the pilot was not using "flight-following services by air traffic control," the NTSB reported.

San Juan County Sheriff Bruce Conrad identified the victims as pilot Harold Joseph Raggio, 72, of Big Bear, California; Steven Dale Wilkinson, 59, of Newberry Springs, California; Rosalinda Leslie, 57, of Hesperia, California; and Michael Lyle Riley, 59, of Barstow, California.