(CNN) — A teen and an adult were killed in a shooting at Central Visual and Performing Arts High School in St. Louis on Monday morning, police Commissioner Michael Sack said.

Officers arrived at the school four minutes after the first 911 call and it took them eight minutes to find the gunman. After a two-minute gun battle, the suspect was reported down, Sack told reporters at a news conference.

Authorities did not immediately identify the victims, but Sack said a 61-year-old woman was pronounced dead at a hospital and a 16-year-old girl was killed at the scene.

The commissioner identified the gunman as Orlando Harris, a 19-year-old who graduated from the school last year. He died at a hospital, Sack said.

Authorities are working to “try and come up with what might have led him to this. There’s suspicion, there may be some mental illness that he was experiencing. We are working on developing that information right now,” Sack said.

The shooter had no prior criminal history.

Seven other teenagers were hurt and went to the hospital, Sack said. Some had gunshot or graze wounds and some had abrasions. One had a fractured ankle. They are all in stable conditions, the commissioner said.

The commissioner said the gunman had long gun and around a dozen 30-round ammunition magazines with him.

Sack would not say how the gunman entered the school but said the school doors were locked, which delayed the suspect and bought responding officers time.

“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,” Sack said.

The St. Louis Police Metropolitan Police Department reported the active shooter on Twitter, and about 45 minutes later, tweeted, “At this time, the scene is secure and there is no active threat.”

The school district is devastated after Monday’s shooting, it said in a statement. CVPA and two neighboring schools were evacuated to a reunification location, St. Louis Public Schools said.

Teacher: Multiple bursts of gunfire

Speaking on the phone, math teacher David Williams said the gunshots erupted shortly after 9 a.m. local time (10 a.m. ET) and everyone went into “drill mode,” turning off lights, locking doors and huddling in corners so they couldn’t be seen.

There was a bang on the door, and it shook, he said. “Someone was trying to open the door,” Williams said.

Sirens emerged in the distance, and then Williams heard three shots, he said. Someone with an adult voice could be heard screaming, “You are all going to f**king die,” he recalled.

Shortly thereafter, a bullet came through one of the windows in his classroom, Williams said.

The gunshots picked up pace, he said.

About then, officers from tactical teams arrived — a huge group of them, well organized — and there was another round of gunshots before Williams heard a woman announcing herself as police, he said.

Williams and the students ran to an emergency exit, he said, adding the ordeal lasted about 40 minutes.

Police engaged the shooter on the third floor, where Williams’ classroom is located, Sack said.

St. Louis Circuit Attorney Kimberly M. Gardner said many people reacted as trained to help save others from being hurt.

“The situation is still developing and we will know more in the coming days, but one thing that is clear is that lockdown procedures — which St. Louis Public School’s administrators, teachers and students at Central Visual and Performing Arts High School and first responders followed as this attack unfolded — were essential in preventing further violence,” Gardner said.

FBI, ATF assisting

The roughly 400-student high school is a magnet school about 6 miles southwest of downtown.

Students were being evacuated from campus “to safe and secure sites,” the district said. People are being asked to avoid the area, and parents have been informed they can pick up their children at Gateway Stem High School, about a mile and a half north of CVPA.

Word of the shooting comes on the same day Michigan teen Ethan Crumbley pleaded guilty to murder charges in a Michigan school shooting last year that left four people dead and seven injured. On November 1, Nikolas Cruz will be sentenced for the February 2018 shooting at Florida’s Marjory Stoneman Douglas High School, where 17 people died.

As the investigation into the St,. Louis shooting was unfolding, a Michigan prosecutor addressed the nation’s gun violence in the wake of Crumbley’s guilty plea.

“It’s not just about sharing with other departments. Gun violence is preventable. That’s what I’ve learned, and the fact that there is another school shooting does not surprise me — which is horrific,” Oakland County Prosecutor Karen McDonald said. “It is preventable, and we should never, ever allow that to be something we just should have to live with.”

During a news conference at the White House, press secretary Karine Jean-Pierre demanded Congress take action and pass an assault weapons ban.

“Our hearts go out to everyone impacted by today’s senseless violence, particularly those injured and killed, their families as well as the first respondents,” she said. “We need additional action to stop the scourge of gun violence.

“Every day that the Senate fails to send an assault weapons ban to the President’s desk, or waits to take another — other common-sense actions is a day too late for our families and communities impacted by gun violence.”

The FBI’s St. Louis field office is assisting local law enforcement in its response to the shooting, spokesperson Rebecca Wu said. The Kansas City, Missouri, field office for the Bureau of Alcohol, Tobacco, Firearms and Explosive is assisting as well, spokesperson John Ham said in a statement.