Originally Published: 24 OCT 22 11:00 ET

Updated: 24 OCT 22 14:24 ET

By Amanda Watts, Caroll Alvarado and Paul P. Murphy, CNN

(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.

Police located the shooter, and he was shot during an exchange of gunfire, he said. He was later pronounced as deceased, Sack said. Police previously said the gunman was in custody.

Authorities did not immediately identify the victims or the shooter, but Sack said during a news conference those slain were a woman who was pronounced dead at a hospital and a teen girl who was killed at the scene. The shooter was a man who appeared to be about 20 years old, he said.

Eight victims were transported to an area hospital, the commissioner said. News of the shooting arrived via social media.

Police arrived within minutes, Sack said, and spoke to students who described the attacker as carrying "a long gun."

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 have been evacuated to a reunification location, and all schools in the district will remain on lockdown for the rest of the day, 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. (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 fking 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. Where the first three shots sounded as if they came from a pistol, the rapid fire of subsequent shots made Williams think they came from a semi-automatic weapon, 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.

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 shooting in St. Louis 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."

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.