The House Energy and Commerce Committee has issued a subpoena to HHS Secretary Sylvia Burwell to turn over documents related to the Affordable Care Act’s reinsurance program, which Republican members on the committee claim has been improperly funded by the agency.

The subpoena follows up on a threat made by the committee in a May letter, which said staff members at HHS and CMS have been “largely unresponsive” to their requests for documentation on the program.

“Congress should not have to issue subpoenas to gain basic facts, but sadly that’s become the case with the Obama administration,” Rep. Fred Upton, R-Mich, chairman of the committee, said in a statement. “Billions of dollars are being illegally diverted, and taxpayers are on the short end. We continue to seek basic information, including the legal justification for these unlawful payments. To withhold such answers is to deny the American people a basic accounting of their tax dollars, and the transparency this administration pledged. Our pursuit of the truth continues.”

The reinsurance program was one of several mechanisms within the ACA aimed at shielding insurance companies from high losses in the first years of open enrollment on ACA marketplaces, and therefore encouraging more insurers to offer plans.

By collecting money from insurers and then awarding it to plans which cover high-risk enrollees, the program was supposed to gather $10 billion in its first year, with $2 billion of that going to the U.S. Treasury. When it fell short of its estimated take, Republicans say HHS reprioritized the Treasury’s share to cover insurer payments.

The committee said it first requested documents from HHS in March, and at an April hearing, CMS Acting Administrator Andy Slavitt said the documents would be provided “soon.”

The May 16 letter from Upton to Slavitt and Burwell said the agency had only provided a six-page document which summarizes a CMS rule, and only mentions the reinsurance program in one paragraph. That letter promised the committee would “use the compulsory process” unless the agency produced the documents by May 30.

House Republicans have successfully challenged funding for other subsidies offered under the Affordable Care Act. In May, a judge sided with the Republicans over HHS in ruling the agency had improperly provided money for cost-sharing reductions for insurers. HHS had said the payments were covered under a permanent appropriation for tax credits to exchange customers, but the judge ruled the cost-sharing program’s funds had to be authorized by Congress.