A 16-year-old gunman killed three children and a teacher at an elementary school in Bend, Oregon, on Thursday before being shot dead by a bystander who pursued him into the building, police said.
The children killed were aged five, eight and 11, while another child and a teacher were critically injured.
Authorities have not established a motive for the attack or determined whether the suspect had connections to the school.
The shooting unfolded at Silver Rail Elementary School shortly before 1 p.m. local time, when police received reports of an armed suspect firing at a vehicle.
Photographs from the scene showed a red truck riddled with bullet holes.
Minutes later, emergency dispatchers received calls reporting that the attacker had entered the school.
Bend Police Chief Mike Krantz said the teenager forced his way through locked school doors by shooting at their locks.
A community member witnessed the attack, followed the gunman into the building and fatally shot him.
Police officers arrived approximately three minutes later and searched the premises.
Authorities identified the suspect as a local 16-year-old boy but withheld his name and the identities of the victims.
The injured seven-year-old child and 54-year-old teacher were transported to hospital in critical condition.
Oregon Governor Tina Kotek praised the bystander's intervention, while local education officials expressed grief over the deaths.
Schools across Bend were closed Friday as the community confronted the aftermath.
The shooting prompted condolences from federal and state officials, including Education Secretary Linda McMahon.
Several lawmakers also renewed calls for stronger gun safety measures.
Silver Rail Elementary serves approximately 450 students and employs more than 40 adults.
According to CNN's analysis, the attack marked the 36th shooting on American K-12 school grounds this year.
The K-12 School Shooting Database, which uses different counting criteria, recorded at least 140 school shooting incidents nationwide this year.