The death of a 4-year-old boy near Mount Spokane last month has triggered a criminal investigation by the Spokane County Sheriff’s Office.

Gerrin P.R. LaFleur died Aug. 19 during a family firewood-gathering trip in the foothills near North Wallis Road and East Elliot Road.

The boy’s father, Lester C. LaFleur, 35, called 911 while driving to a fire station at North Market Street and East Farwell Road and said he thought his son was dead, but investigators have heard conflicting witness reports about what happened, according to a search warrant filed Monday in Spokane County District Court.

That warrant authorized detectives to scour LaFleur’s 1985 Ford F-250 truck for hair fibers, fingerprints and other evidence in the vehicular homicide investigation.

LaFleur, who could not be reached for comment, has not been named as a suspect.

It’s unclear if the truck was moving when Gerrin died. LaFleur said the boy had fallen from the top of the truck, but an autopsy showed he died from a bruised and cut right lung, according to the search warrant.

“This injury was not consistent with just a 7- to 8-foot fall alone,” wrote Detective David Thornburg. “… The truck had to be moving.”

Doctors at Providence Sacred Heart Medical Center declared the boy dead. Thornburg noted LaFleur wasn’t at the hospital and “initially refused” to cooperate with the investigation.

When he arrived, he had three of his other children with him, ages 6, 8 and 10, who were on the trip when Gerrin died, Thornburg wrote.

The oldest child told Thornburg her brother was riding inside the cab with LaFleur when he “got hit by a branch and was pulled from the vehicle while it was moving.”

Two days after the trip, Gerrin’s uncle told a TV news station the boy had unbuckled his seat belt and climbed out of the truck.

Detectives spotted two child-sized boot marks on the top of the driver’s side rear bumper when photographing LaFleur’s truck the day of Gerrin’s death, according to the warrant.

Sheriff’s spokesman Sgt. Dave Reagan said the investigation is ongoing.

“Every death we investigate, we investigate as a homicide until we know it’s something else,” Reagan said. “If you go in assuming it’s accidental, you can blind yourself to the possibility that there’s evidence to recover.”