"I just watched the light come out of the sky," witness Mike Krause told The Billings Gazette. "To me it looked like the plane was upside down. And then it just came straight down into the lot."
The plane appeared to hit the ground in the lot of a rental business and skid into the construction materials building, witnesses said.
Duane Demars, who lives nearby, said he heard two loud explosions and four smaller ones.
"It sounded excruciating," he said. "When I got here, flames were at a length of 200 to 300 feet and 20 feet high in the air."
The construction materials building and plane were destroyed by the blaze, and a nearby United Rentals building was damaged by debris, Fergus said.
Alpine Air, which owned the plane, confirmed it was carrying mail bound for Great Falls.
A Postal Service worker arrived to secure the mail, but it was not clear whether any survived the fire.
Fire officials confirmed the pilot's body was found in the wreckage. His name was not immediately released.
Airport fire chief Michael Glancy said at least 300 gallons of jet fuel spilled in the crash, creating puddles that caught fire.
