A Washington state teenager whose grandmother foiled his plot to shoot up a school last year has been sentenced to 22 years in prison.

Snohomish County Superior Court Judge Bruce Weiss handed down the sentence against Joshua O'Connor, 19, who pled guilty in December to first-degree attempted murder, first-degree robbery and possession of an explosive device in connection with the plot.

O'Connor's plot to shoot up ACES Alternative High School last year was discovered by his grandmother, Catherine Katsel, who called 911 after she found journal entries made by her grandson that stated he wanted to shoot other students at his high school. The teenager was arrested one day before the shooting at Marjory Stoneman Douglas in Parkland, Florida.

The judge called Katsel a hero, and thanked her for taking action to avoid the potential tragedy.

"I truly believe, even though it was argued that your grandson could have changed his mind and not carried out his plan on that day, I’m convinced, based on everything that I saw, that he was going to carry it out," Weiss said. "Had you not intervened, there would have been many, many lives that would have been lost, including your own grandson’s."

Katsel showed officers excerpts from her grandson's journal, which read in part:

"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,"

"I need to make this shooting/bombing ... infamous. 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."

Katsel told police that she'd discovered a semi-automatic rifle hidden in a guitar case in O'Connor's room. Police believe the teen also used the rifle at one point to rob a gas station of $100.

Judge Weiss told O'Connor that he hoped the teenager would use the time to follow through with his plans furthering his education and writing a biography to serve as a cautionary tale for others.

"You’re still going to be relatively young by the time you get out, and you do not have to let the mistakes that you’ve made in the past define you as a person the rest of your life," said Weiss. "That choice is going to be up to you, though."

"There is no child that is a throwaway," the judge said. "You are not a throwaway. You are an important person just like everybody else. You made some very poor choices and poor decisions. Hopefully you can learn from that experience and move forward and have a productive life."

In a written statement delivered to the court, O'Connor apologized for his actions writing that he had been suicidal and abusing drugs and alcohol.

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