The 16-year-old gunman who shot and killed three children and one teacher at Silver Rail Elementary School in Bend, Oregon, on Thursday has been identified as John Griffin Jr. III, two law enforcement sources briefed on the investigation told CBS News. Police say he was shot and killed by a bystander at the school.

The attacker used a shotgun purchased by his mother, law enforcement sources told CBS News. 

A motive for the attack has not been provided. The suspect was from Bend, but police have not said if he had a connection to the school. 

Law enforcement sources told CBS News that investigators are combing through his computer, phone, social media, search history and other online activity. They are probing whether the shooter may have been obsessed with violence, including prior mass shootings. The sources emphasized that the investigators are in the preliminary stages of their investigation. 

The children killed in the shooting were ages 5, 8 and 11, Bend Police Chief Mike Krantz said during a news conference Thursday night. 

The 5-year-old victim has been identified as Rhoam Summer, according to fundraisers and a social media post from his father. 

Two other people — a 7-year-old child and a 54-year-old teacher — were hospitalized with critical injuries. 

Krantz said the gunman entered the school by “shooting at locked doors.” He was pursued by a bystander, who engaged the shooter and shot and killed him, Krantz said. 

The family of the bystander issued a statement Friday identifying him as Kaylub Giamanco, a Bend resident with two daughters who attend Silver Rail Elementary School. 

“He is not seeking recognition, but instead wishes to express his gratitude to the first responders, school personnel, and community for the outpouring of love and support,” the family said.

Police said they responded within minutes to the 911 calls from the school and found the deceased shooter and other victims inside.