 -- A grandmother in Washington state is being credited with reporting her own grandson for planning a school shooting. The teen was arrested the same day as a former student killed 17 people in a shooting at a Florida school.

ABC News obtained a 911 call from a grandmother alerting police that her grandson, who was arrested Wednesday, was in the advanced stages of plotting a shooting at his high school.

“What I’m reporting is I’m finding journal entries from my grandson,” Catherine O’Connor told the operator. “And he’s planning on having a mass shooting at one of the high schools.”

O’Connor’s 18-year-old grandson, who has not been identified by police, was a student at Aces Alternative High School in Everett, Washington.

ABC's Seattle affiliate KOMO obtained court documents detailing his violent journal entries.

“I need to make this shooting/bombing ... infamous," the student wrote, according to the court documents. "I need to get the biggest fatality number I possibly can. I need to make this count. ... I'm learning from past shooters/bombers mistakes, so I don't make the same ones."

The grandmother called police when she discovered her grandson’s journal as well as a semi-automatic rifle stored in a guitar case in her home, according to court records.

Another journal entry stated, "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) away."

His journal also had excerpts about making pressure cooker bombs.

The teen had considered targeting more than one high school, according to one of his journal entries. Everett police arrested the grandson at ACES High School on Wednesday. KOMO reported he was in possession of marijuana and a knife at the time of his arrest, and was booked for third-degree assault on an officer for kicking him during his arrest.

He is being held on $5 million bail at Snohomish County Jail. Police are still investigating the teen for attempted murder based on his journals.

“There's a lot of kids who could have lost their lives,” O’Connor’s neighbor Julie Primachik told KOMO. “I think it’s a hero-like thing to do.”