Mom, 3-year-old son found dead in remote wooded area after boy's dad arrested for murder: Officials

Karissa Fretwell, 25, and her 3-year-old son, Billy, went missing in May.

The bodies of an Oregon mother and her 3-year-old son have been recovered together, authorities said, weeks after they went missing and the little boy's father was arrested for murder.

Karissa Fretwell, 25, of West Salem, and her son William Fretwell, who went by Billy, were last seen on May 13 and relatives reported them missing on May 17, the Salem Police Department said.

Billy's father, Michael John Wolfe, 52, of Gaston, Oregon, was arrested in May and charged with aggravated murder and kidnapping, police said.

On Saturday searchers found the bodies of Karissa Fretwell and Billy in a heavily wooded and very remote area of Yamhill County, about 10 miles west of the city of Yamhill, according to a joint statement from the Yamhill County Sheriff's Office, Salem Police Department and Yamhill County District Attorney.

Wolfe is familiar with that area, Yamhill County District Attorney Brad Berry said at a news conference Monday.

Karissa Fretwell died from a single gunshot to her head, authorities said, and the death was ruled a homicide.

Billy's cause and manner of death are not yet known and require more testing to be determined, authorities said Sunday. Billy's manner of death is expected to be a homicide, Berry said.

The bodies were found together and were hidden, Berry said.

Karissa Fretwell and Billy were kidnapped the night of May 13, Berry said. Investigators are "confident" the mother and son were dead by May 16, Berry said.

Authorities have not released any potential motive. Berry said Monday that there was "no other person who had, as we could tell, anything to gain" from Karissa Fretwell's death besides Wolfe.

Authorities have not ruled out the possibility of another suspect's involvement, Berry said.

Wolfe is married, said Berry, and Karissa Fretwell had sole custody of her son, according to the sheriff's office.

Wolfe is set to be arraigned on Thursday. His attorney did not immediately respond to ABC News' request for comment.