Some Kmart stores were targeted by hackers, leading to unauthorized activity on some of its customers’ credit cards, the retailer’s parent company said. Sears Holdings Corp. said in a blog post late Wednesday that Kmart’s store payment systems were recently infected with virus-like computer code undetectable by current anti-virus systems. It said some credit card numbers were stolen. A Sears Holdings spokesman said the investigation into the hack is still ongoing, so details on the dates of the breach, how many customers were affected and which stores were targeted, were not available. Not all Kmart stores were affected, he said. Kmart had 624 stores at the end of April.