The truck and body were found Saturday on private property down a long driveway in the 7300 block of Woods Creek Road, about nine miles north of Monroe and about six miles south of Lake Roesiger.
Lt. Susy Johnson of the Snohomish County Sheriff's Office said deputies were dispatched to the scene at about 3:30 p.m. Saturday after receiving a 911 call.
A Snohomish County sheriff's patrol car is parked at the entrance to the driveway where the body was found.
She said investigators with the Major Crimes unit decided to wait until daybreak Sunday to begin physically gathering evidence at the property.
"This is still being treated as a suspicious death at this time," she said.
A sign at the entrance to the private driveway warns: "No Trespassing. Violators Will Be Shot. Survivors Will Be Shot Again."
The burned-out truck was found backed into some trees on the property, and the body was found outside.
When deputies arrived at the scene Saturday, it was clear that the fire in the truck had died out some time before. However, investigators were not immediately able to say how long the truck and body have been there.
Officials did not say whether the body is a man or woman, nor have they revealed the cause of death.
A residence is located on the property, but no one was home when authorities arrived.