By Jim Finkle BOSTON (Reuters) – Michaels Companies Inc, the biggest U.S. arts and crafts retailer, said it is investigating a possible security breach on its payment card network and advised customers to check their financial statements for fraudulent activity. If confirmed, it would mark the second known data breach since 2011 at Michaels, which is preparing to sell shares in an initial public offering. “We are concerned there may have been a data security attack on Michaels that may have affected our customers’ payment card information,” Michaels Chief Executive Chuck Rubin said in a statement on Saturday.