MARYLAND HEIGHTS, Mo. (AP) — A part-time janitor killed a co-worker at a crowded suburban St. Louis community center before he was shot and seriously wounded by a police officer, authorities said Tuesday.

The gunman was upset about being sent home when he opened fire Monday night, killing on-duty supervisor Maria Lucas, 45, said Maryland Heights police Chief Bill Caron. Frantic patrons ran for cover, some of them hiding in closets while clutching weights to toss at the gunman.

An officer was in the parking lot when a woman ran outside and told him about the shooter. The officer then rushed inside and exchanged gunfire with the shooter, hitting him multiple times.

Carson didn't release the name of the gunman, who was in serious condition. It wasn't clear why he had been sent home.

The officer was not injured.

The center houses a police substation, and Carson said the uniformed officer had been there earlier, filling out a police report. He described the officer's actions as heroic, saying he didn't hesitate or wait for backup.

Sasha Kondratyeva told the St. Louis Post-Dispatch that she was working out and saw the shooter frantically pacing in the entry to the building with a gun before hearing a series of loud shots.

Everyone on the second floor then began running to the back rooms, and Kondratyeva hid with about 20 people in a utility closet, where some called family members to tell them they loved them and parents fretted about children that were in the center's childcare facility. Some men in the closet picked up small weights to use to hit the shooter if if he tried to enter the closet, Kondrateyeva said.

"I thought this could be it," she said.

The city said the community center would be closed until further notice.