According to The Associated Press, a new report released by police on Friday reveals that 18-year-old Karl Pierson, who shot and killed 17-year-old Claire Davis at Arapahoe High School in Centennial, Colorado, on December 13, 2013, wanted revenge for teasing. 

“Words hurt, can mold a sociopath, and will lead someone a decade later to kill,” Pierson wrote in his diary, according to police.

Arapahoe County Sheriff David Walcher said that Pierson was upset at debate coach Tracy Murphy for removing him as captain on September 3, 2013.

Reuters reports that the 18 year old entered the school, armed with a shotgun, a machete and several Molotov cocktails, and shot Davis, who was waiting outside the library with a friend. Once inside the library, Pierson demanded to know where his debate coach was. According to the report, Pierson saw his debate coach and shot twice, but missed. Murphy escaped unharmed and Karl took his own life before police could take him into custody.

Excerpts of Pierson’s diary, obtained by The Denver Post, reveal that Karl was obsessed with December 13, 2013. On Saturday, October 26, 2013, he wrote, “the 13th of December is a great date, as the 347th (47 is a great number) date of the year, there are 18 (my age) days left. It is a day of gore, filled with murder, suicide.”

Several weeks later, he mentioned the December 13 date again, writing, “I am ecstatic right now. That December 13 date I chose is perfect, it is 38 days after the fifth of November. I love that date, everything about it.”

The report also revealed, according to Reuters, that Pierson was evaluated for mental health issues several times before the shooting, but was not flagged as a threat to the school or himself.

“I will do something I have wanted to do for a while – mass murder and be in a place of power where I and I alone are judge, jury and executioner,” Pierson, who researched school shootings on his computer, wrote.