The U.S. labor department has subpoenaed Tribune Co. in an investigation connected to the stock-ownership plan that was a key feature of Sam Zell's 2007 deal to take Tribune private, the Wall Street Journal is reporting. Tribune disclosed the investigation in a bankruptcy court filing on Thursday and calls it a routine request for documents. Zell's buyout deal "involved a complicated structure under which the [employee stock ownership plan] became the majority owner of the company, a feature that helped Tribune avoid corporate taxes," the WSJ story says.