SAN FRANCISCO -- A man and a woman were shot dead on Saturday in a dispute over "guns or drugs" in the State of Washington on the US west coast, local authority said.

Pierce County Sheriff's Department said on Facebook that sheriff deputies responded to calls about shots fired and they found bodies of a 35-year-old man and a 25-year-old woman at a mobile home in Key Peninsula, about 48 km southwest of Seattle, the largest city of Washington state.

The couple's bodies were found in the driveway of the mobile home near two vehicles, and an initial probe showed that the shooting was related to an argument over guns or drugs.

The victims' two-year-old son was in the back of their vehicle in the driveway area during the shooting, but he was not injured, the Sheriff's Department said.

He has been placed with a relative who does not live on the property, the police said.

Police officers saw a 27-year-old male suspect in the nearby area and placed him into custody, the Sheriff's Department said.

Police believed the suspect was involved in a recent domestic violence assault.

Pierce County, the second most populous county in Washington state, has seen rising gun- and drug-related crimes in recent years.

By 2016, about 38 percent of the methamphetamine labs cleaned up by the Washington Department of Ecology were in Pierce County.