This hits close to home, and what Mr. O’Connor’s grandmother did is probably the best way to prevent mass shootings:

A would-be school shooter in Everett bought inert grenades, hid a military-style rifle in a guitar case and carried out an armed robbery to fund an elaborate plot to kill his classmates, according to police.

The grandmother of Joshua Alexander O’Connor, 18, found alarming journal entries Tuesday at her home on Holly Drive, according to reports filed in court. She called police. An officer pulled O’Connor from class at ACES High School to arrest him.

“I’m preparing myself for the school shooting,” he had written in the journal. “I can’t wait. My aim has gotten much more accurate … I can’t wait to walk into that class and blow all those (expletives) away.”

O’Connor wrote that he wanted the death count to be as high as possible so that the shooting would be infamous, according to court papers. He went into detail about building pressure-cooker bombs, activating inert grenades and deploying explosives for maximum casualties.

“I need to make this count,” O’Connor reportedly wrote. “I’ve been reviewing many mass shootings/bombings (and attempted bombings) I’m learning from past shooters/bombers mistakes.”

To her shock, the grandmother found a semiautomatic rifle in O’Connor’s guitar case. She did not know he had a rifle. She called police at 9:25 a.m. Tuesday.

The grandmother showed officers excerpts from the journal. The teen had written about flipping a coin to choose if he would target Kamiak High School or ACES, the alternative school he transferred to in October. His school was picked in the coin flip.

His grandmother is brave. By turning her grandson in, she risks retribution by Mr. O’Connor if or when he gets out of the slammer. But that’s how it’s done. In recent history, in the county where I live, there have been two other mass shootings, so we’re unfortunately well aware of the “see something, say something” mantra.

Mass shootings on and off campus have left scars in Snohomish County, too. In October 2014, a freshman at Marysville Pilchuck High School killed four teenagers — Shaylee Chuckulnaskit, Andrew Fryberg, Zoe Galasso and Gia Soriano — before taking his own life.

A man, 20, killed his former Kamiak High School classmates, Anna Bui, Jake Long and Jordan Ebner, at a house party in July 2016. The shooter was sentenced to life in prison.

Anyway, I’m glad that ACES High School shooting won’t be added to the database. At minimum, the prevention of this potential mass shooting will save us from yet another diatribe on Morning Joe on banning semiautomatic rifles.