An acid spill at a New York City office building early Monday injured six people, including four FDNY firefighters. One person was seriously injured in the incident.

Hazardous material teams responded to the spill at around 9:30 a.m. at the 21-story Yorkshire Towers in Manhattan, and it took crews more than three hours to handle the situation, WNBC-TV reports. FDNY officials believe muriatic acid—commonly known as  hydrochloric acid—from the building's swimming pool was the toxic chemical released in the incident, but it's unclear if the acid was spilled in the water or somewhere outside of the pool.

Details concerning the injuries to the firefighters and other people were not released. The person with serious injuries was the only individual taken to the hospital, according to WNBC.