Lewis County court records show the 15-year-old pleaded guilty Tuesday to four charges of child molestation and four charges of raping a child.
The 11-year-old victim in the case told authorities that state Sen. Brian Hatfield's son had abused him at Hatfield's home in Raymond and at another home in Chehalis, according to law enforcement documents.
The records say that Hatfield became aware of the abuse in mid-February and had worked to keep the boys apart but did not report the matter to authorities.
Lewis County investigators first began examining the issue two weeks ago after school officials reported that a student had disclosed details of sexual abuse.
Cristine Beckwith, a lawyer for Hatfield, said in a statement that Hatfield called the attorney's office after learning of the inappropriate behavior to see what the best course of action would be.
She said the lawmaker was in the process of setting up appropriate counseling for his son and that the son was then planning to turn himself in to authorities.
John Meyer, the prosecuting attorney in Lewis County, said he did not expect charges would be filed against Hatfield for failing to report the abuse because Hatfield is not required to report under state law.
"There's no basis to file any charges," Meyer said.
David Burke, the prosecutor in Pacific County where other incidents occurred, said he wants the case handled entirely in Lewis County, in part because Burke personally knows Hatfield and because it would be easier for one prosecutor to handle it all.
He said he is prepared to appoint Lewis County officials as special deputy prosecutors if they decide to pursue charges for incidents in Pacific County.
Hatfield, a Democrat, first joined the Legislature in 1994 and was re-elected last year to the state Senate, where he is a leader on issues relating to agriculture and rural economic development. He and his wife have four children, and the law enforcement documents say the couple maintains residences in both Pacific County and Lewis County.
Beckwith said Hatfield's son has had several catastrophic losses in his life, including the death of his biological mother at a young age.
"His extremely difficult childhood is a contributing factor in this case, and is not an uncommon history of other juvenile offenders," Beckwith said in a statement.