House Oversight and Government Reform Committee Chairman Darrell Issa, R-Calif., subpoenaed the Treasury Department on Friday to force production of documents on IRS political targeting that the agency has refused to hand over.

“While the Obama administration has so publicly deflected responsibility for the targeting, it simultaneously has attempted to thwart congressional oversight into the matter,” Issa wrote in a letter to Treasury Secretary Jacob Lew. “The IRS has engaged in a systematic effort to delay, frustrate, impede, and obstruct the committee’s investigation.”