RITZVILLE, Wash. (AP) — The state Attorney General's Office says a man has pleaded guilty to murdering his girlfriend on Interstate 90 in front of their 3-year-old son last March.

Manuel Argomaniz-Camargo entered the plea Tuesday and is due to be sentenced Oct. 4 in Adams County Superior Court. Investigators said he bludgeoned Ana Montelongo Garcia with a hammer and stabbed her with a screwdriver on I-90 near Ritzville.

The Attorney General's Office handled the case at the request of the Adams County Prosecutor's Office. Attorney General Bob Ferguson says his office is seeking a sentence of nearly 30 years in prison.

The defendant pleaded guilty possession of methamphetamine in addition to first-degree murder.