A pump malfunction at the Flowood, Mississippi, YMCA swimming pool sent 12 children to the hospital.

According to the Flowood Police Chief Rickey McMillian, the incident occurred at a Sunday afternoon birthday party when something went wrong with the pump.

“From what I’m told, it was a pump issue that after the chlorine was poured into the pool, the pump wasn’t circulating it, and they realized it right before the party, which caused extra fumes,” he said.

Emergency personnel responded to reports of children choking and vomiting.

Rebecca Day, a mother of one of 12 children rushed to the emergency room, posted on Facebook that all of the children are ages 7 and under. Day said that her child was continuing to be monitored at the hospital the following day.

“We are one of the families in the E.R.,” Day wrote. “We shouldn’t have been allowed in the pool if there was a problem.”

Day wrote that the children were swimming for 15 to 20 minutes before their symptoms started.

“Shortly after swimming, all of the kids started choking and couldn’t catch their breath for a moment,” Day posted. “My son vomited five times.”

YMCA personnel said that the pool is equipped with automatic chlorine pumps. It remained closed the following day as a third party planned to inspect what happened.

They stated their records showed the chlorine levels were fine.