EVERETT, Wash. -- Authorities in Washington arrested an 18-year-old man on Tuesday who was said to be planning an attack on a school. The alleged plan was foiled by his grandmother.

Joshua O'Connor may have been the next school gunman. But then his grandmother, Cathi O'Connor, called 911.

"I'm finding journal entries from my grandson," Cathi told 911 dispatchers. "He's planning on having a mass shooting at one of the high schools."

Cathi called police when she discovered a note in her grandson's journal that read in part: "I'm preparing myself for a school shooting. I can't wait. My aim has gotten much more accurate."

"It couldn't have been an easy thing for her to do, but boy, it was sure the right thing for her to do," said Andy Muntz of the Mukilteo School District.

Joshua Alexander O'Connor KIRO

Court documents say the 18-year-old had a semi-automatic rifle hidden in a guitar case, bomb making equipment and "detailed plans to shoot people at Aces High School" where he was a student.

"I need to get the biggest fatality number I possibly can," O'Connor wrote.

Authorities say the rifle matches one used Monday to rob a convenience store, and that O'Connor wrote in his journal "about how powerful he felt and how scared the female cashier was at him pointing his gun at her."

Prosecutors believe the robbery was meant to raise money for more material for the attack his grandmother prevented.