Claiming that it has turned over “only a fraction” of the requested documents, House Oversight and Government Reform Committee Chairman Darrell Issa, R-Calif., sent a subpoena today to Treasury Secretary Jacob Lew regarding the 2009 taxpayer-funded $50 billion bailout of General Motors.

“Taxpayers deserve the full truth about how decisions were made to use their money in an Administration effort that resulted in protecting generous pensions for unionized Delphi employees while greatly diminishing benefits for non-unionized employees,” Issa said in the letter. The chairman has been investigating the matter since 2010.

For over a year, Treasury has slow-walked his document requests, he claimed. “This continued lack of cooperation has left little choice but the issuance of subpoena to compel the production of documents.”

Issa is looking into evidence that the Treasury Department, not the Pension Benefit Guarantee Corporation, was the “driving force” behind the decision to favor the unionized employees.

According to a 2012 study by the conservative Heritage Foundation this decision may have resulted in the unionized employees getting $26 billion more than they would have otherwise gotten under regular bankruptcy rules.