What downed a Beechcraft Bonanza airplane last week and killed two men remains a mystery, after an examination of the engine could not determine mechanical malfunction.

The airplane crashed into a semi parked on South Ninth Street at 9:59 a.m. July 3, with David Gibson, 61, of Aspen and Larry Smalley, 65, of Rifle aboard. The men died almost instantly of multi-systems trauma, including burns, Montrose County Coroner Mark Young said last week.

The weather was clear at the time of the crash and the condition of the plane?s propeller led National Transportation Safety Board investigators to initially suspect it wasn?t properly powered. The NTSB took the engine and wreckage to a Greeley salvage yard for further examination.

?There was nothing mechanically wrong with the engine,? NTSB Senior Air Safety Investigator Arnold Scott said Monday. He later also said there was no discernible issue with the propeller. ?That leaves us up in the air right now. We know it came down because of a loss of power; why it lost power, we don?t know.?

He said he found nothing wrong with the engine components that were still extant.

?There are some components we can?t look at because they were destroyed by fire,? he said. Those components included the fuel pumps and fuel manifold, along with most of the wiring.

Scott said at a press conference last week investigators hadn?t seen anything wrong with the weather and that both Smalley and Gibson were qualified pilots; accordingly, the NTSB was probing the possibility of engine failure. The investigation is ongoing and no possible cause has been completely ruled out.

Gibson, an Aspen-area architect with a business in Telluride, was flying with Smalley, a flight instructor, in order to log flight training time in the recently purchased Beechcraft for insurance purposes.

It was not known for certain who had actually been flying the plane when it crashed; however, Scott reported Gibson had been found in the left seat, while Smalley was in the right seat. The aircraft had only one flight control yoke, but can be flown from either side. Scott said investigators examined the control wheel and determined it was in the left seat position.

Gibson and Scott had apparently been performing landing and takeoff maneuvers at the Montrose County Regional Airport and were heading due southeast when the plane lost power and plunged through trees into the semi.

Witness reports as to how the plane came down varied, with Art Gutierrez reporting it?d ?cartwheeled? after clipping the Levi Hawks home at 543 S. Ninth St. It hit the rig and burst into flames, killing both men.

?It was nosed into that semi,? eyewitness Brian Yocum said Monday. ?All I could see was the top of the plane and the right side. I couldn?t even tell you where that plane came from, other than straight down, right beside me.?

Yocum was hauling liquid oxygen for Hartman Brothers at the time of the crash. He?d just turned off of Park Avenue onto Ninth Street, and glanced at the semi as he drove by.

?Right when I was getting ready to pass it, I heard this tremendous crash. I stopped my truck. I looked up to see a fuselage out my side window. The glass out of that semi was all over my work truck.?

Yocum quickly backed away from the site because of the fire danger. He made it 20 yards before the first explosion occurred, which he said totally engulfed the truck and the plane. ?It?s almost like it melted away from the truck,? he said. The Beechcraft then pivoted away, fell to the ground and a second explosion rang out.

Yocum said the plane, while it was still embedded in the semi, ?was almost vertical.?

?The heat just seemed to melt that plane down. It started to cant, almost rotate and then it just laid down in the street.?

Young, who is also a pilot, said that based on the plane?s path, it appeared the men were trying to glide to the Columbine Athletic Field near the intersection of Park Avenue and 12th St., but did not have sufficient altitude.

Scott said the investigation could be completed in as few as six months.