A 16-year-old male fatally shot three children and a teacher as he forced his way through locked doors at Silver Rail Elementary School in Bend, Oregon, on Thursday. Another child and a teacher sustained critical injuries.

The suspect, identified as a Bend resident, was found dead inside the school minutes after the shooting was first reported. Authorities stated that a civilian who pursued the shooter into the school engaged and killed him. The first 911 call came at approximately 1 p.m. from a person in a vehicle outside the school who reported being shot at with a long gun. Subsequent calls originated from within the school building.

The victims killed were children aged 5, 8, and 11. The injured 7-year-old child and 54-year-old teacher were transported to a hospital in critical condition.

Police said they are investigating whether the shooter knew anyone at the school or had previously attended.

The attacker used a pump-action shotgun that had been purchased by his mother, according to a law enforcement official familiar with the investigation who spoke on the condition of anonymity.

The school, located about 125 miles southeast of Portland, serves kindergarten through fifth grade students and had 459 enrolled students as of the previous year. All classes, sports, and extracurricular activities for the 32 schools in the Bend-La Pine school district were canceled for Friday.

Oregon Governor Tina Kotek expressed her condolences, calling her heart “absolutely broken” and commending the civilian who intervened to prevent further casualties.

This incident marks the 28th mass killing in the U.S. this year, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University.

Children were reunited with their parents at a nearby church, which served as a reunification center.