New details are surfacing about how a 16-year-old gunman armed himself before killing three children and a teacher at Silver Rail Elementary School in Bend, Oregon — and the central fact investigators are grappling with is painful and simple: the shotgun belonged to his mother.

An unnamed law enforcement official told the Associated Press, in reporting carried by Newsweek and IBTimes UK, that the pump-action shotgun used in last Thursday's attack was purchased by the shooter's mother. Police have not publicly confirmed that detail themselves, and investigators say they are still working to pin down exactly how and when the teenager, identified as Griffin J. John II, obtained access to the weapon.

A Former Student Returns

John, 16, was a former student at Silver Rail Elementary, according to CTV News, which cited Bend police. He killed three children and a teacher on October 15 before being shot and killed himself, officials said; an unidentified community member reportedly saw him enter the building, followed him and engaged him before officers arrived to find him already dead.

The shooter's mother, Shauntai Nickerson, spoke briefly to reporters in the attack's aftermath. She said if there had been anything she could have done to prevent it, she would have done it, according to East Idaho News's coverage of the case, but she declined to say how her son ultimately obtained the gun.

IF THERE WAS ANYTHING I COULD HAVE DONE, I WOULD HAVE DONE IT

Under Oregon law, a minor may lawfully possess a shotgun if it was provided by a parent, or by another adult with a parent's consent — meaning the purchase itself, as described, would not necessarily have violated state statute even as it has become the focal point of public anger over how the massacre was allowed to happen.

A Community Still Grieving

The names of the three children and the teacher killed in the attack were released by police in the days following the shooting, and vigils have continued across Bend as the small Central Oregon city works through the aftermath of the deadliest school shooting the state has seen in years. Investigators have not disclosed a motive, and the school district has said grief counselors remain available to students, staff and families as classes resume at other campuses.

No charges are expected against Nickerson at this time; officials have described the investigation into the weapon's path into her son's hands as ongoing. The Bend Police Department said it would provide updates as the inquiry progresses, but cautioned that some details — including the full circumstances of how the shotgun ended up in the teenager's possession — may not be resolved for weeks.