The incident happened Friday morning at a Unique Thrift store, the Chicago Tribune reported.
A store employee, whose name was not reported, told police he found a sock with something inside while sorting through clothes. When he emptied the sock into his hand, a loaded .22-caliber gun fell out and accidentally discharged, striking his coworker, Carmen Dominguez, 54, in the chest.
Dominguez was taken to a hospital where she was pronounced dead.
"Preliminary indications during the course of the investigation are that this is a tragic, accidental incident," police said in the statement.
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