EVERETT, Wash. -- Authorities arrested a Washington state student suspected in a school shooting plot after his grandmother showed officers plans for an attack.

Police in Everett, which is about 30 miles north of Seattle, arrested 18-year-old Joshua Alexander O'Connor on attempted murder and other charges at ACES High School on Tuesday, The Daily Herald reported.

Earlier, O'Connor's grandmother called 911 and showed responding officers a journal where he allegedly drew up plans to shoot students and use homemade explosives at the school, police said.

O'Connor's entries included details about making pressure-cooker bombs, activating inert grenades and deploying explosives for maximum casualties, court papers said.

"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."

The journal entries, obtained by CBS affiliate KIRO, continued: "I'm preparing myself for the school shooting. I can't wait. My aim has gotten much more accurate. … I can't wait to walk into that class and blow all those [expletive]s away."

O'Connor also allegedly wrote of making the attack "infamous" and talked of getting "the biggest fatality number I possibly can," KIRO reported.

Detectives serving a search warrant at the grandmother's house seized the student's journal, a rifle found inside a guitar case and inert grenades.

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