Police Commissioner Michael Sack said on Tuesday that the 19-year-old shooter who killed a teacher and a 15-year-old student at a St. Louis high school was equipped with an AR-15-style weapon and what seemed to be more than 600 rounds of ammunition.

In addition, Orlando Harris left a handwritten note explaining why he carried out the shooting at Central Visual and Performing Arts High School on Monday. Seven pupils were hurt, including tenth grader Alexandria Bell and physical education instructor Jean Kuczka, who was 61.

In a gunfight, Harris was killed by police.

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Harris wrote a letter, which Sack read, lamenting his lonely life and lack of friends, family, and a girlfriend. He described it as the “perfect storm for a mass shooter” in the note.

Sack claimed Harris had magazines placed in stairwells and some ammunition strapped to his chest and in a backpack.

The attack forced students to barricade doors and huddle in classroom corners, jump from windows and run out of the building to seek safety. One terrorized girl said she was eye-to-eye with the shooter before his gun apparently jammed and she was able to run out. Several people inside the school said they heard Harris warn, “You are all going to die!”

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Harris, 19, graduated from the school last year. The FBI was assisting police in the investigation. Sack, speaking at a news conference, urged people to come forward when someone who appears to suffer from mental illness or distress begins “speaking about purchasing firearms or causing harm to others.”

Relatives of those killed mourned their losses.

“Alexandria was my everything,” her father, Andre Bell, told KSDK-TV. “She was joyful, wonderful and just a great person.”

Alexandria, a 10th grader, was outgoing, loved to dance and was a member of the school’s junior varsity dance team.

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“She was the girl I loved to see and loved to hear from. No matter how I felt, I could always talk to her and it was alright. That was my baby,” Andre Bell said.

Abby Kuczka said her mother was killed when the gunman burst into her classroom and she moved between him and her students.

“My mom loved kids,” Abbey Kuczka told the St. Louis Post-Dispatch. “She loved her students. I know her students looked at her like she was their mom.”

All seven of the injured students are between the ages of 15 and 16. All of them had stable conditions listed. Sack said that one had a fractured ankle, two had bruises, four had gunshot or grazing wounds, and four had evidently jumped out of the three-story structure.