An investigation into the Feb. 24 accident that killed 4-year-old Mason Geist near Spring Valley revealed there was no mechanical malfunction in the U-Haul truck involved in the accident. Douglas County Undersheriff Paul Howell said investigators believe the boy shifted the truck from park into neutral, setting it in motion and resulting in Mason being bounced out of the cab. Family members took video of the truck shifting from park to neutral without the brake pedal being pressed. They said a label on the truck’s dash told drivers the brake had to be pressed to work the shift. “I think …