The 19-year-old gunman who killed a teacher and a 15-year-old girl at a Missouri high school was armed with an AR-15-style rifle and what appeared to be more than 600 rounds of ammunition, police said.

Orlando Harris also left behind a hand-written note offering his explanation for the shooting on Monday at Central Visual and Performing Arts High School in St Louis.

Tenth-grader Alexzandria Bell and 61-year-old PE teacher Jean Kuczka died and seven students were wounded.

Police killed Harris in an exchange of gunfire.

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

The commissioner said Harris had some ammunition strapped to his chest, some in a bag, and other magazines were found dumped in stairwells.

The attack forced students to barricade doors and huddle in classroom corners, jump from windows and run out of the building to seek safety. Several people inside the school said they heard Harris warn, “You are all going to die!”

Harris, 19, graduated from the school last year. The FBI was assisting police in the investigation. Mr 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.

“Alexzandria was my everything,” her father, Andre Bell said. “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 all right. That was my baby.”

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

“My mom loved kids,” Abbey Kuczka said. “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. All were said to be in a stable condition. Police 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.

Harris was armed with nearly a dozen 30-round high-capacity magazines, Mr Sack said.

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

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

Monday’s St Louis shooting came on the same day a Michigan teenager pleaded guilty to terrorism and first-degree murder in a school shooting that killed four students in December 2021.