A carbon monoxide leak has killed two people in a New Jersey recording study and injured 12 others.

Police responded to a 911 call at around 1:30 p.m. Saturday at the Streets Rehearsal Studio in Passaic, which lies north of Newark and is part of the New York City metro area, according to Fox News.

Police found two people dead on the second floor of the building, which is three stories tall. Another 12 victims experienced “confusion and were choking” as they tried to leave the building, which is a mixed-use commercial building. They are receiving treatment at local hospitals, and their identities were not released.

Calling it a “sad day” for Passaic, the city’s mayor, Alex Blanco, praised the reaction of emergency personnel, which he said saved additional lives from being lost.

The cause of the poisonings is under investigation, but authorities believe that carbon monoxide is the most likely culprit. A city spokesman told a local newspaper that mixed-use commercial buildings aren’t required to have carbon monoxide detectors.

The studio has been open since 1994 and offers 300 to 400 square foot studios for musicians who want to “hone their craft,” according to the studio’s website.

Carbon monoxide is an odorless and tasteless invisible gas that is toxic to humans when it rises above a certain concentration. It is produced in small quantities in animal metabolism.