EVERETT, Wash. — A grandmother foiled a possible mass shooting at her grandson’s Pacific Northwest high school this week, authorities reported a day after the massacre in Parkland, Florida.

The woman, who has not been named, called 911 Tuesday morning when she read a journal reportedly written by 18-year-old Joshua Alexander O’Connor, and came across what police say was a plot to carry out a mass shooting at ACES High School in Everett. She also found a semiautomatic rifle hidden in his guitar case.

Police later arrested O’Connor at the school.

Authorities say the teen detailed how he planned to shoot students and detonate homemade explosives. The devices, police allege, are pressure-cooker bombs like those used in the 2013 attack at the Boston Marathon. O’Connor was also in possession of empty hand grenades he allegedly planned to fill with gun powder.

“I’m preparing myself for the school shooting,” he allegedly wrote. “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. I need to make this count. I’ve been reviewing many mass shootings/bombings (and attempted bombings) I’m learning from past shooters/bombers mistakes.”

It’s also been alleged that O’Connor aimed for the death count “to be high so he would be infamous.”

According to reports, O’Connor attended another school, Kamiak High School in Mukilteo, Washington, before transferring to ACES last October. Detectives say he flipped a coin to decide which school to target and ACES won the toss.

“We are really grateful to the grandmother,” Mukilteo School District spokesman Andy Muntz told the International Business Times. “It couldn’t have been easy for her to do.” It speaks to the importance of the saying, ‘if you hear something or see something, call authorities,’ and that’s what she did. It’s quite possible she saved many lives including her grandson’s.”

O’Connor is charged with suspicion of attempted murder. He’s being lodged in the Snohomish County Jail on a $5 million bond. R.