(CNN) — The 19-year-old gunman who killed two people and wounded several others at his former high school left a note saying his struggles led to “the perfect storm for a mass shooter,” St. Louis police said.

Orlando Harris graduated from Central Visual and Performing Arts High School last year. He returned Monday with an AR-15-style rifle, over 600 rounds of ammunition and more than a dozen high-capacity magazines, St. Louis police Commissioner Michael Sack said.

Harris died at a hospital after a gun battle with officers.

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Investigators found a handwritten note in the car Harris drove to the school. Sack detailed some of the passages:

“I don’t have any friends. I don’t have any family. I’ve never had a girlfriend. I’ve never had a social life. I’ve been an isolated loner my entire life,” the note said, according to Sack. “This was the perfect storm for a mass shooter.”

Given the gunman’s extensive arsenal, the tragedy could have been “much worse,” the police chief said.

Authorities credited locked doors and a quick law enforcement response — including by off-duty officers — for preventing more deaths at the school.

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But the shooter did not enter a checkpoint where security guards were stationed, said DeAndre Davis, director of safety and security for St. Louis Public Schools.

Davis also said the security guards stationed in the district’s schools are not armed, but mobile officers who respond to calls at schools are.

“For some people that would cause a stir of some sort,” Davis said Tuesday. “For us, we thought it’s best for our officers, for the normalcy of school for kids, to not have officers armed in the school.”

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Student Alexandria Bell, 15, and teacher Jean Kuczka, 61, were gunned down in the attack.

One of the teacher’s colleagues, Kristie Faulstich, said Kuczka died protecting her students.

During the rush to evacuate students from the school, “One student looked at me and she said, ‘They shot Ms. Kuczka.’ And then she said that Ms. Kuczka had put herself between the gunman and the students,” Faulstich said.

Kuczka was looking forward to retiring in just a few years, her daughter Abigail Kuczka told CNN.

Alexandria was looking forward to her Sweet 16, her father Andre Bell told CNN affiliate KSDK.

“It’s a nightmare,” Bell said. “I am so upset. I need somebody — police, community folks, somebody — to make this make sense.”

He joins a growing list of parents grappling with the reality of their child being killed at school.

Across the country, at least 67 shootings have happened on school grounds so far this year.

As the shooting unfolded in St. Louis, a Michigan prosecutor who just heard the guilty plea of a teen who killed four students last fall said she was no longer shocked to hear of another school shooting.

“The fact that there is another school shooting does not surprise me — which is horrific,” Oakland County Prosecutor Karen McDonald said.

“We need to keep the public and inform the public … on how we can prevent gun violence. It is preventable, and we should never ever allow that to be something we just should have to live with.”

‘How did that man get inside the school?’

Bell, the father of the slain teen, said he’s struggling to get answers about what happened.

“I really want to know: How did that man get inside the school?” he told KSDK.

Authorities have said the doors were locked. But the St. Louis police commissioner declined to detail how the shooter got in.

“I don’t want to make this easy for anybody else,” Sack said.

The gunman didn’t conceal his weapon when entering the school, Sack said.

“When he entered, it was out … there was no mystery about what was going to happen,” the commissioner said. “He had it out and entered in an aggressive, violent manner.”

‘Miles Davis is in the building’

Faulstich said school’s principal came over the intercom and used the code phrase “Miles Davis is in the building” to let faculty know an active shooter was in the building.

“I instantly but calmly went to lock my door and turn off the lights,” the teacher said. “I then turned to my kids and told everyone to get in the corner.”

Within a minute of locking her second-floor classroom door, Faulstich said, someone started “violently jostling the handle, trying to get in.”

“I absolutely commend my students for their response,” Faulstich said. “Even in the moments when they were hearing gunfire going on all around they stood quiet and I know they did it to keep each other safe.”

Adrianne Bolden, a freshman at the school, told KSDK that students thought the school was conducting a drill — until they heard the sirens and noticed their teachers were scared.

“The teacher, she crawled over and she was asking for help to move the lockers to the door so they can’t get in,” Bolden said. “And we started hearing glass breaking from the outside and gunshots outside the door.”

Sophomore Brian Collins, 15, suffered gunshot wounds to his hands and jaws. He escaped by jumping from a classroom window onto a ledge, his mother VonDina Washington said.

“He told me they heard an active shooter notification over the intercom so everyone in the class hid,” Washington said. According to her son, the gunman then came into the classroom and fired several shots before leaving.

After the gunman left the third-floor classroom, Washington said another student opened a classroom window, and some of them jumped.

Brian has numbness in his hands and trouble moving some of his right-hand fingers.

“He’s really good at drawing,” Washington said. “He went to CVPA for visual arts, and we’re hoping he’ll be able to draw again.”

Math teacher David Williams told CNN everyone went into “drill mode,” turning off lights, locking doors and huddling in corners so they couldn’t be seen.

He said he heard someone trying to open the door and a man yell, “You are all going to f**king die.”

A short time later, a bullet came through one of the windows in his classroom, Williams said.

His classroom is on the third floor, where Sack said police engaged the shooter.

Eventually, an officer said she was outside, and the class ran out through nearby emergency doors.

Officers rushed in about 4 minutes after the shooting started

Security personnel were at the school when the gunman arrived, St. Louis Public Schools Communications Director George Sells said.

“We had the seven personnel working in the building who did a wonderful job getting the alarm sounded quickly,” Sells said.

The commissioner did say the school doors being locked likely delayed the gunman.

“The school was closed and the doors were locked,” Sack told CNN affiliate KMOV. “The security staff did an outstanding job identifying the suspect’s efforts to enter, and immediately notified other staff and ensured that we were contacted.”

After widespread controversy over the delayed response in confronting school shooters in Uvalde, Texas, and Parkland, Florida, Sack said responding officers in St. Louis wasted no time rushing into the school and stopping the gunman.

“There was no sidewalk conference. There was no discussion,” Sack said. “There was no, ‘Hey, where are you going to?’ They just went right in.”

A call about an active shooter at the high school came in around 9:11 a.m., according to a timeline provided by the commissioner.

Police arrived on scene and made entry four minutes later.

Officers found the gunman and began “engaging him in a gunfight” at 9:23 a.m. Two minutes later, officers reported the suspect was down.

Asked about the eight minutes between officers’ arrival and making contact with the gunman, Sack said “eight minutes isn’t very long,” and that officers had to maneuver through a big school with few entrances and crowds of students and staff who were evacuating.

Police found the suspect “not just by hearing the gunfire, but by talking to kids and teachers as they’re leaving,” Sack said.

As phone calls came in from people hiding in different locations, officers fanned out and searched for students and staff to escort them out of the building.

Officers who were at a church down the street for a fellow officer’s funeral also responded to the shooting, the commissioner said.

A SWAT team that was together for a training exercise was also able to quickly load up and get to the school to perform a secondary sweep of the building, Sack said.

Some officers were “off duty; some were in T-shirts, but they had their (ballistic) vests on,” the commissioner said. “They did an outstanding job.”

Correction: An earlier version of this story gave the wrong age for 15-year-old Alexandria Bell, who was killed in the shooting.

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