SEATTLE (Reuters) - A 3-year-old boy in Washington state was shot in the mouth apparently by a 4-year-old playmate, law enforcement officials said.

The boy had been playing with his neighbor late Friday at his family's Lake Stevens home, 35 miles north of Seattle, when his "father heard a loud noise coming from the bedroom," a Snohomish County Sheriff's Office statement said.

The wounded boy's father told officers the bedroom door was locked and he forced it open, finding his son with a mouth injury. The father said he thought the boy might have been hurt when the door was forced open, the sheriff's statement said.

The boy was rushed to a nearby hospital where "it was determined that the three-year-old had a gunshot wound to the mouth," the statement said. The child was transferred to a hospital in Seattle where he was in serious condition on Saturday, officials said.

The kind of firearm involved was not yet known, the sheriff's office said.

"Initial information at the scene suggested that the four- year-old neighbor may have accidentally shot the three-year-old while playing, although the cause of the shooting is still under investigation," the statement said.

The families of both children are cooperating with the investigation, the sheriff's office said.

Last year, a 4-year-old New Jersey boy fatally shot his 6-year-old playmate with his father’s .22-caliber rifle in his parents' bedroom. Anthony Senatore, 35, pleaded guilty in October to multiple counts of child endangerment for failing to secure weapons in the home.

(Reporting by Victoria Cavaliere in Seattle; Editing by Grant McCool)