Karissa Fretwell, 25, and her 3-year-old son, William, were identified Sunday.
Their bodies were found Saturday in an isolated area 15 km west of the small town of Yamhill in northwestern Oregon, officials said in Salem, Oregon, and Yamhill County.
William's father, Michael Wolfe, 52, was indicted last week for the kidnapping and murder of William and Fretwell, NBC affiliate KGW reported.
The state medical examiner found that Fretwell had been killed by a single shot in the head.
The cause of William's death has not yet been determined, the statement said.
Fretwell and William were last seen on May 13, according to KGW.
Citing court documents, the station reported that Wolfe and Fretwell appeared to be involved in a custody dispute. Wolfe has recently been ordered to pay Fretwell $ 900 a month for child support, according to the channel.