A Matteson firefighter died Friday afternoon while performing CPR on a shopper at a Walmart in south suburban Matteson.

Mark Zielinski, 49, was “rendering emergency medical aid” to a shopper who needed CPR at the Walmart at 21410 S. Cicero Ave. in Matteson when he died, according to Matteson Fire Chief Chris Schwalbe and the Cook County medical examiner’s office.

He had been with the fire department for thirteen years after spending many years at the Harvey Fire Department, Schwalbe said.

An autopsy conducted Saturday found that Zielinski, of the 500 block of Miller Street in Beecher, died of natural causes, suffering from coronary artery thrombosis and coronary atherosclerosis, according to the medical examiner’s office. Obesity was listed as a secondary cause of death to the heart problems.

Zielinski was taken to Franciscan St. James Health Hospital in Olympia Fields, where he was pronounced dead at 3:45 p.m., the medical examiner’s office said.