ST. LOUIS (AP) — A 19-year-old who killed a teacher and a 15-year-old girl at a St. Louis high school was armed with an AR-15-style rifle and what appeared to be more than 600 rounds of ammunition, a police official said Tuesday.

Orlando Harris also left behind a handwritten note offering his explanation for the shooting Monday at Central Visual and Performing Arts High School, St. Louis Police Commissioner Michael Sack told reporters. Tenth-grader Alexzandria Bell and 61-year-old physical education teacher Jean Kuczka died and seven students were wounded before police killed Harris in an exchange of gunfire.

Sack read Harris’ note in which the young man lamented that he had no friends, no family, no girlfriend and a life of isolation. Harris called it the “perfect storm for a mass shooter.”

Sack said Harris had ammunition strapped to his chest and in a bag, and that additional magazines were found dumped in stairwells.

“This could have been much worse,” Sack said.

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 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!”

Harris graduated from the school last year. 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.”

Alexzandria was a bright, charismatic girl with a sassy personality who was working hard to improve her dancing and her grades, said Central’s principal, Kacy Seals-Shahid. She was a member of the school’s junior varsity dance team, her father said.

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

“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,” he said.

The morning of the shooting, Alexzandria’s mom brought her daughter’s glasses to the school when she noticed the teenager had left them home. Her mom got to the school before Alexzandria arrived by school bus.

“When Alex got off her bus, I asked her, `Aren’t you going to need these because you can’t see without those?” Seals-Shahid said. “The family was super supportive of Alexzandria.”

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.”

The seven injured students are all 15 or 16 years old. Sack said four suffered gunshot or graze wounds, two had bruises and one had a broken ankle — apparently from jumping out of the three-story building. All were listed in stable condition.

The school in south St. Louis was locked, with seven security guards at the doors, St. Louis Schools Superintendent Kelvin Adams said. A security guard initially became alarmed when he saw Harris trying to get in one of the doors. He had a gun and “there was no mystery about what was going to happen. He had it out and entered in an aggressive, violent manner,” Sack said.

That guard alerted school officials and made sure police were contacted.

Harris managed to get inside anyway. Sack declined to say how, saying he didn’t want to “make it easy” for anyone else who wants to break into a school.

Police offered this timeline: A 911 call came in at 9:11 a.m. alerting police of an active shooter. Officers — some off-duty wearing street clothes — arrived at 9:15 a.m.

Police located Harris at 9:23 a.m. on the third floor, where he had barricaded himself inside a classroom. Police said in a news release that when Harris shot at officers, they shot back and broke through the door.

At 9:25 a.m., when Harris pointed his rifle at police, they fired several shots. He was secured by police at 9:32 a.m.

Police said Alexzandria was found in a hallway and died at the scene. Kuczka was found in a classroom and died at a hospital.

Central Visual and Performing Arts shares a building with another magnet school, Collegiate School of Medicine and Bioscience. Central has 383 students, Collegiate 336.

It was the 40th school shooting this year resulting in injuries or death, according to a tally by Education Week — the most in any year since it began tracking shootings in 2018. The deadly attacks include the killings of 19 children and two teachers at Robb Elementary School in Uvalde, Texas, in May.

Matt Davis, president of the St. Louis Board of Education, said police and school officials acted quickly to Monday’s shooting.

“And yet, we are still left with tragedy,” Davis said.

For now, the survivors are dealing with the trauma.

The gunman pointed his weapon at Raymond Parks, a dance teacher at the school, but did not shoot him, Parks said. The kids in his class escaped outside and Parks stopped traffic and get someone to call the police. They came quickly.

“You couldn’t have asked for better,” Parks said of the police response.

Ashley Rench said she was teaching advanced algebra to sophomores when she heard a loud bang. Then the school intercom announced, “Miles Davis is in the building.”

“That’s our code for intruder,” Rench said.

The gunman tried the door of the classroom but did not force his way in, she said. When police officers started banging, she wasn’t sure at first if it really was law enforcement until she was able to glance out and see officers.

“Let’s go!” she told the kids.

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Salter reported from O’Fallon, Missouri. Associated Press writers Margaret Stafford and Heather Hollingsworth in Kansas City, Missouri, contributed to this report.