A security guard fatally shot a man Thursday afternoon while trying to break up a fight outside a retail store in South Shore, Chicago police said.

Just before 3 p.m., a man, 25, was arguing with another man, 22, in the 7100 block of South Jeffery Boulevard when one of the men drew a gun and started shooting, police said. A security guard working at a retail store nearby then pulled out his own gun and fired shots, striking the 25-year-old.

The gunman was taken to University of Chicago Medical Center, where he was pronounced, police said.

The younger man suffered a gunshot wound to the hip and was taken to the same hospital, where he was listed in critical condition, police said.

No other details were immediately known.