(CBS Local) — One day before 17 people were killed in a Florida school shooting, a grandmother helped to stop another massacre in Washington state.

According to police in Everett, Washington, the grandmother of Joshua Alexander O’Connor called 911 after finding disturbing journal entries that described how the 18-year-old was planning to attack his high school.

“Officers were also told the grandson had a rifle stored in a guitar case. As officers reviewed copies of the journal, they were alarmed at the statements and detailed plans to shoot students and use homemade explosive devices at ACES High School,” a statement from Everett police read.

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

Detectives, serving a search warrant on Feb. 13, reportedly found an AK-47 and inert grenades in the 18-year-old’s bedroom. O’Connor was booked into the Snohomish County Jail and has reportedly been charged with attempted first-degree murder, felony assault on an officer, and first-degree robbery.

“Our main thing right now is gratitude, especially to the grandmother,” Andy Muntz of the Mukilteo School District said. “That couldn’t have been easy for her to do. The Everett police also did a wonderful job. That combination may have saved a lot of lives.”

The grandmother’s act of heroism took place nearly 24 hours before 19-year-old Nikolas Cruz brought an AR-15 rifle into Marjory Stoneman Douglas High School, killing 17 and injuring more than a dozen on Feb. 14.