CNN
 — 

The gunman who killed two people and wounded several others in a school shooting in St. Louis, Missouri, on Monday was flagged by an FBI background check but was still able to purchase the AR-15-style rifle he used in the attack from a private seller, police said.

When 19-year-old Orlando Harris first tried to purchase a gun from a licensed dealer, the background check blocked the sale, St. Louis Metropolitan Police Sgt. Charles Wall said Thursday. But Harris could still legally buy the rifle from a private individual who had bought the firearm from a licensed dealer in 2020, Wall said.

Harris’s family had been worried about his mental health, so when his mother found the rifle in their home, the family contacted police, authorities said.

Missouri does not have a so-called “red flag law” which would allow police to confiscate a person’s gun if they are at risk of causing harm to themselves or others. So St. Louis police arranged for Harris’s rifle to be given to “a third party known to the family” so it could be stored outside the home, police said in a statement to CNN affiliate KMOV.

Yet somehow, when the teen forced his way into the Central Visual and Performing Arts High School on Monday morning, he had the rifle back in his hands.

Armed with the high-powered firearm and an arsenal of over 600 rounds of ammunition and more than a dozen high-capacity magazines, the shooter opened fire into the hallways of the school, which he had just graduated from last year.

As students and teachers scrambled to lock and barricade doors and take shelter, he continued his rampage, fatally shooting talented student Alexandria Bell, 15, and beloved teacher Jean Kuczka, 61, and wounding multiple others.

Within minutes, officers had arrived at the school and quickly engaged the shooter in a gunfight, according to St. Louis Police Commissioner Michael Sack. Harris was later pronounced dead at a local hospital.

Police are working to determine how the shooter regained possession of the rifle, Sack said Wednesday.

School officials were given access to the bullet-riddled building on Tuesday, but it could be weeks or months before students are brought back to the Central Visual and Performing Arts and Collegiate School of Medicine and Bioscience high schools, which share a campus, St. Louis Public Schools Superintendent Dr. Kelvin Adams said Tuesday.

“Obviously with the kinds of things that happened in our building, we need to make sure that the building is ready to receive students and staff and the community, as well,” Adams said. He noted counseling services are available for students and staff.

The attack on the St. Louis high school is at least the 67th shooting to happen on American school grounds this year, marking another devastating moment in the growing reality of gun violence against students and educators.

Witnesses of the shooting describe a horrifying scene in which the school learned there was an active shooter in the building through a coded message announced over the intercom.

As soon as history teacher Kristie Faulstich heard the announcement, she knew what to do.

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

Teachers and law enforcement have applauded how students conducted themselves during the attack.

“We’ve had teenagers and athletes – they don’t always listen – but on Monday they sure did,” Sack said Wednesday. “They did what their teachers instructed them to do, they do what the officers instructed them to do, despite the fact that you can see that many of them were traumatized. You can see their faces, you can read in their eyes.”

“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.”

Several students escaped the building by leaping from windows, students and teachers have said.

There were seven security personnel at the school when the gunman arrived, but he did not enter the building through a checkpoint where security guards were stationed and instead had to force his way in, according to DeAndre Davis, director of safety and security for Saint Louis Public Schools.

Police officers arrived at the school within four minutes of the active shooter being reported, according to Sack, who has repeatedly credited swift law enforcement response, locked doors and training for preventing further deaths.

“The fact that it takes this level of response to stop a shooting like this because people have access to these weapons of war and can bring them into our schools can never be normal,” said St. Louis Board of Education President Matt Davis.

The school district has been working to add gun safety to the curriculum, Superintendent Adams said at a press conference Tuesday.

“The gun safety initiative, quite frankly, was a plan put together to try to address the kind of issues that happen outside of our school district, outside of our school buildings, in terms of the number of students who have been shot in the city of St. Louis, and that die, quite frankly, as a result of incidents that happened outside of the school environment,” Adams said.

“Never did I think I would be standing here today having a conversation about a staff (member) and a student” being shot, Adams said, pausing to keep composure as his voice began to break.