Police officers fatally shot the suspect before further damage could be done but the incident saw two victims killed and seven injured. Police Commissioner Mike Sack held an evening news conference in which he said that it was not immediately obvious what the motive was behind the violent attack.
During the news conference, Mr Sack noted that the suspect may have suffered from mental health issues.
The Commissioner said that as the police officers arrived at the scene, students could be seen fleeing the Central Visual and Performing Arts High School.
According to Sack, the students informed officers that the gunman was armed with a rifle.
The emergency calls to the police came in at around 9.10am local time (3.10pm BST) and officers were able to end the horror attack just 15 minutes after they were alerted.
There was an exchange of gunfire between the suspect and the officers after they stormed the building in which the suspect was fatally shot.
Witnesses of the attack have said that lives had been saved as the gunman’s rifle jammed during the attack.
Taniya Gholston, 16, told the St Louis Post-Dispatch newspaper that the attacker entered the classroom she was in and tried to shoot her.
However, the weapon jammed, giving her time to escape.
She said: “I was trying to run, and I couldn’t run. Me and him made eye contact but I made it out because his gun got jammed.”
Mr Sack noted that the gunman was found carrying hundreds of bullets which were in almost a dozen high-capacity magazines meaning “this could have been much worse”.
The Commissioner informed reporters that the suspect was Orlando Harris, 19, who had completed his final year at the school last year and had no prior criminal record.
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A 61-year-old teacher and a 16-year-old girl were killed during the incident.
Seven others were injured, with four teenagers sustaining gunshot wounds and three others receiving injuries from the panicked students attempting to flee.
According to The St Louis Post-Dispatch, the teacher was identified as Jean Kuczka, who was a teacher of health and physical education.
The Police Commissioner said: “While on paper we might have nine victims…we have hundreds of others.
“Everyone who survived this is going to take home trauma.”