Chief Criminal Deputy Jim Chamberlain said authorities have not yet identified the woman, but the case is being investigated as a homicide.
The driver, identified as 26-year-old Bernard K. Howell III, of Tenino, was booked into the Thurston County Jail for investigation of second-degree murder and first-degree theft. Monday afternoon, a judge set bail at $750,000.
Erik Kohnhorst told KOMO News Howell showed up on his Tenino home doorstep Sunday afternoon, agitated, covered in sweat and desperate.
"He was trying to get me to help him move something and I asked him if it was heavy and stuff and he said 'yeah,' " Kohnhorst said. "His clothes were kind of messed up."
Kohnhorst later surmised Howell was asking him to help move the body.
"That's the only thing I can figure," Kohnhorst said.
Hours later, investigators say a man spotted Howell along Highway 507 near the Yelm to Tenino trail, by himself, loading a body into his truck.
Police say Howell asked the man to help put the body in the passenger seat because the back of his truck was filled with a large, white refrigerator he used to store meat.
Truck driven by Bernard Howell
The man refused, drove into town and flagged down a deputy. The deputy spotted the truck a short time later and stopped the driver near Tenino Elementary School.
Chamberlain said the man admitted to the deputy he had a body in his pickup and said he found the body along the trail and was trying to save the family the cost of a funeral.
Witness Jerry Jones said he watched as officers pulled the victim's body out of the truck, wrapped in plastic and a sleeping bag.
"You could see between the truck and rear wheel, here this person was laying," Jones said.
Investigators say they found the victim's wallet and keys under the front seat. Also, a vehicle found in the area belongs to a Yelm woman who has not been seen since Sunday morning, but Chamberlain said investigators don't know for sure if that is the same person found dead in the truck.
Officials also said they are looking into whether Howell may be connected to the disappearance Nancy Moyer, who vanished from Tenino last year.
The 36-year-old mother of two was last seen in March 2009. Her front door was found open, with all her belongings still inside and her car was in the driveway.
Detectives say they have nothing at this point linking Howell to the case, but they are not ruling anything out and plan to ask him about Moyer.
In the meantime, investigators have released a photo of the truck Howell was driving and are asking anyone who saw this truck in the Tenino area on Sunday is asked to give the Thurston County Sheriff's office a call.
Prosecutors say Howell has a history of mental illness. Howell faces another court hearing Thursday where prosecutors hope to charge him then send him to Western State Hospital for a full evaluation.