Officers responded to the scene, a home in the Peninsula area of Moses Lake, just before 10 p.m. Friday after receiving a 911 call.
Arriving at the residence, they found two adults with gunshot wounds. Detectives also recovered a gun believed to have been used in the shootings.
The Columbian Basin Herald identified the two adults as the boy's parents. Both victims were transported to an out-of-area hospital for treatment.
The names of the victims were not released.
According to school officials, the boy is a student at Moses Lake Christian Academy. A dance scheduled for Saturday has been canceled because of the incident.
School officials released a statement Saturday on their Facebook page:
"We don't have much information to share right now, but we want to acknowledge that one of our students was involved in an off-campus shooting last night. The student was the shooter and he is in police custody.
"There are two victims who are hospitalized. We do not have information on their conditions, and in coordination with the Moses Lake Police Department, we will wait for the MLPD to release specific information on the investigation and the alleged crime.
"Our information will be coordinated with Moses Lake Police to make sure the investigation's integrity is sustained. We are directing the media to contact Moses Lake Police for information specific to the investigation.
"There is no risk to other students or the school.
"We are extremely saddened by this incident and we are praying for the student and the victims. We are a small school and everyone is extremely close, which is a blessing because that is one of the qualities which will help us all get through this tragedy.
"In addition, the MLCA high school dance which was scheduled for tonight has been cancelled in respect for those involved in the shooting incident."
The Moses Lake Police Department is continuing to investigate the shooting.