The U.S. Department of Labor subpoenaed Tribune Co over an employee stock plan crucial to real estate mogul Sam Zell's buyout of the media company, Tribune said in a bankruptcy court filing on Thursday.

The subpoena, which the Labor Department issued in March, asks for an extensive range of documents related to the newspaper publisher and broadcaster's employee stock ownership plan, or ESOP, Tribune said in the filing.

Tribune handed over the documents on March 31, it said.

We view this as a routine inquiry, Tribune spokesman Gary Weitman said in an e-mail message.

The Labor Department's questions relate to the Employee Retirement Income Security Act, a law designed to protect people who participate in employee retirement plans.

The filing did not say what kinds of documents the government was seeking or what it planned to do with them.

The stock ownership plan was an important piece of Zell's plan to take Tribune private in an $8.2 billion deal that involved $13 billion in debt.

The plan was designed to let the ESOP become the largest owner of the Chicago-based company, which would let it avoid corporate taxes. That, in turn, was supposed to help Tribune turn around its financial performance as advertising sales at U.S. newspapers tumble.

Tribune, which owns the Los Angeles Times, Chicago Tribune, The Sun in Baltimore and other U.S. papers, ended up filing for bankruptcy last December.

Some current and former Tribune employees sued last year over the ESOP, saying that Zell and Tribune have mismanaged the company.

Tribune also is facing other legal questions.

Earlier this week , Connecticut Attorney General Richard Blumenthal sent a letter to Zell warning him that Tribune's consolidation of operations at two TV stations in Connecticut and the Hartford Courant daily newspaper could violate federal media ownership rules.

Tribune also is involved in the case of former Illinois Governor Rod Blagojevich, who reportedly tried to get Tribune to fire editorial page staffers who were critical of him in exchange for helping Tribune find state financing in a deal to sell Wrigley Field.

Tribune owns the field as well as the Chicago Cubs, the professional baseball team that calls Wrigley home.

(Reporting by Robert MacMillan; Editing by Jan Paschal)