(CBS) – A firefighter in the south suburb of Matteson died suddenly Friday while responding to a medical emergency, officials said.

The 49-year-old firefighter responded to the scene of a Matteson retail store, where a shopper required CPR, Fire Chief Chris Schwalbe said in a prepared statement Friday evening. During the call to render assistance, the 13-year department veteran died.

The Cook County Medical Examiner’s Office identified the firefighter as Mark Zielinski of Beecher. He previously worked for the Harvey Fire Department.

It wasn’t immediately known what happened to the original medical patient.