The chemical accident Monday at the New Rochelle YMCA was caused when staff mixed chlorine and hydrochloric acid by mistake, creating a noxious gas that spread through the aquatics facility, according to an email from Chief Executive Officer Jenn Ryan.

The New Rochelle Police Department reported after the accident Monday that 13 people had been sent to area hospitals.

“We remain in contact with Jacobi Medical Center, White Plains Hospital and Montefiore New Rochelle Hospital on the status of these individuals and are offering our full support to them,” said Ryan.

The building remained closed Tuesday, though the New Rochelle Fire Department ruled it was safe to return, the email said.

Agencies responding to the accident included the New Rochelle, Larchmont and Pelham Manor fire departments, New York State Department of Environmental Conservation, Westchester County Health Department and the New Rochelle Building Department.

“The safety and security of everyone who enters our doors is our top priority,” said Ryan. “We will conduct a thorough investigation into this incident to better ensure nothing like it happens again.”