EVERETT, Wash. (AP) — A former Washington state student has pleaded guilty to criminal charges after his grandmother reported his plans to carry out a mass shooting at his school.

On Thursday, 18-year-old Joshua Alexander O’Connor entered a new guilty plea to attempted first-degree murder, unlawful possession of an explosive device and robbery.

O’Connor was arrested in February after his grandmother found his journal where he detailed plans to kill as many people as possible at ACES High School in Everett.

Authorities say O’Connor stockpiled weapons, wanted to be as infamous as other mass shooters and wrote in detail about making pressure-cooker bombs, activating inert grenades and deploying explosives for maximum casualties.

Police stopped him the day before he was to carry out the shooting.

O’Connor now faces up to 28 years in prison.