MARYLAND HEIGHTS, Mo. (AP) — A part-time janitor killed a worker at a suburban St. Louis community center crowded with around 150 people, before an officer who was outside the building rushed in and opened fire on the gunman, possibly preventing a much larger tragedy, authorities said Tuesday.

The 30-year-old gunman had just been told he was being sent home from work Monday night “and he reacted with anger,” killing 45-year-old Maria Lucas, the building’s acting supervisor, Maryland Heights police Chief Bill Carson told reporters Tuesday.

A patron ran out and found a uniformed officer in the parking lot. The officer ran into the building, engaged the shooter in a gun battle, leaving the shooter seriously wounded, Carson said. The officer was not hurt.

“It’s my belief that the actions of the officer involved were absolutely heroic,” Carson said. “He immediately went and sought out the threat. His actions may have prevented further bloodshed. We don’t know right now the intentions of the gunman.”

The name of the shooter has not been released pending formal charges.

Carson declined to name the officer but said he graduated from the police academy just 14 months ago.

The shooting happened shortly after 8 p.m., when the center was busy with people exercising — pickleball, karate classes, swimming and other activities were going on. Carson declined to say why the janitor was being sent home or if Lucas, a customer service representative who was working the front desk while also serving as the acting supervisor, was involved in the decision.

Carson said the worker confronted Lucas at the front desk, then pulled out a 9 mm semiautomatic handgun and shot her in the head.

Several other people near the front desk scattered, while the shooter started pacing back and forth, according to Carson.

The officer had just exited the building after finishing paperwork at a police substation inside the community center.

When a woman ran out and told him what had happened, “the officer immediately and without hesitation went inside,” Carson said.

The gunman saw the officer coming through a glass door and opened fire, shattering the glass but missing the officer. The officer and gunman then exchanged several shots. The gunman was struck multiple times before becoming incapacitated, Carson said. The officer was not hit.

Carson described a chaotic scene as the shooting unfolded. Employees helped children get out or find a hiding place. Adults took refuge in closets and darkened rooms, some clutching weights as weapons in case the shooter found them.

Police spent hours after the shooting searching to make sure everyone was accounted for.

Sasha Kondratyeva told the St. Louis Post-Dispatch that she was working out when she heard 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 child care facility.

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

Carson said investigators were still reviewing the officer’s body camera as well as footage from security cameras inside the center. He planned to meet with St. Louis County Prosecuting Attorney Wesley Bell later Tuesday in anticipation of charges.

City Council member Susan Taylor recalled Lucas as “sweet and funny.” She said she once ran into Lucas at a pharmacy and their conversation was so joyful they ended up talking in the aisle for 30 minutes.

“So when I heard last night that that person is gone ... that’s really hard,” Taylor said.