(CHICAGO) Five Chicago Fire Department firefighters were taken to the hospital early Monday when they reported “feeling ill” after battling an earlier blaze.

About 3:20 a.m., the firefighters were all taken from the Engine 26 Firehouse, 10 N. Leavitt St., according to Fire Media Affairs Chief Juan Hernandez.

Three firefighters were taken to Stroger Hospital, one was taken to Rush University Medical Center and a fifth firefighter was transported to Saints Mary and Elizabeth Medical Center, Hernandez said. They were all listed in fair-to-serious condition.

The nature of their illness and which fire may have caused it or contributed to it remains under investigation early Monday.