HILO — An investigation into the crash of an Island Hoppers tour plane on Mauna Loa last year failed to find any mechanical cause.

The release of the federal National Transportation Safety Board's "factual report" is a preliminary step in determining the cause of the June 17, 2008, crash that killed a pilot and two visitors.

According to the report, "no evidence of any mechanical discrepancies was found with the airplane's airframe or engine that would have prevented normal operation."

An autopsy of the pilot, 40-year-old Katsuhiro Takahashi of Kailua, Kona, found no traces of drugs in his system, except for those used to treat high blood pressure. He died upon impact from brain trauma. Takahashi's passengers were Masako and Nobuhiro Suzuki of Japan.

Senior NTSB air safety investigator Jim Struhsaker said the final determination of probable cause would come from the board in Washington, D.C., in two to six weeks.

Struhsaker cannot determine the cause of the crash. But the factual report bolsters the theory that human error, as opposed to mechanical failure, was to blame.

The day of the crash, the Cessna 172M had taken off from Kona International Airport under a counter-clockwise flight plan that was supposed to take it north to Upolu Point, south to South Point and then back to Kona along the shoreline. The flight was estimated to take 2 1/2 hours, with the aircraft flying at 2,000 feet.

During a standard pre-flight weather briefing, Takahashi was told to expect scattered clouds at 3,000 feet, with broken to overcast clouds at 5,000 feet and visibility at 5 miles with moderate rain showers.

Another Island Hoppers aircraft near Kalapana was the last to see the Cessna. A witness who was just a mile away from the crash site reported hearing the plane's "deep rumble" that suddenly stopped. He said the weather was very foggy, misty and rainy.

Five days later, fire rescue personnel finally arrived at the crash scene on the southern slope of Mauna Loa, at an elevation of 4,500 feet some nine miles from the coast.

The track of downed vegetation leading to the airplane indicated that it had been headed west-northwest.

Struhsaker wondered why the plane had deviated so far off its flight plan.

Struhsaker's investigation also focused on the engine of the 35-year-old plane. The engine's crankshaft, valve train, spark plugs, combustion chambers and exhaust system components all appeared normal.