Wheeling’s pension plan is under investigation.

The Pension Benefit Guaranty Corp, which insures retirement incomes for a third of the American work force, has begun investigating the pension plan supporting 10,000 retirees at Wheeling-Pittsburgh Steel. The steelmaker, unable to meet payments on $500 million owed to its lenders, filed Tuesday for protection under Chapter 11. The filing jeopardizes the pension plan, which is already underfunded by an estimated $200 million and could collapse in September, when the company will be unable to meet an obligation to deposit $60 million, according to Joseph Scalise Jr., a Wheeling-Pittsburgh vice president.