Authorities arrested an 18-year-old in Washington State Tuesday whose grandmother discovered a journal with detailed plans to shoot his high school classmates, police said.

Police arrested Joshua Alexander O’Connor, 18, at ACES High School on Tuesday on multiple charges, including attempted murder, the Associated Press reported.

O’Connor’s grandmother called 911 Tuesday morning after noticing a journal that allegedly detailed plans to shoot students and detonate homemade explosives at the school.

She showed the responding officers portions of the journal, which explained plans to make pressure-cooker bombs and deploy explosives with the intent of causing mass casualties, court documents state.

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

“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,” the alleged writings continued.

Detectives conducting a search at the grandmother’s residence seized the journal, inert grenades, and a rifle placed inside a guitar case.

O’Connor is being held in Snohomish County Jail on $5 million bond.

The student’s arrest came the day before 17 people were killed in a shooting at Parkland High School in Parkland, Florida. Police arrested 19-year-old Nikolas Cruz, a former student at Parkland High School, accused of carrying out the massacre.