Two House Republican committee chairmen have issued a subpoena for information about how the Department of Health and Human Services is funding an Obamacare program without congressional approval, after HHS has refused to provide the information.

"We've been trying to get answers since last June, but HHS simply will not cooperate," Ways and Means Committee Chairman Kevin Brady, R-Texas, said in a joint statement with Energy and Commerce Committee Chairman Fred Upton, R-Mich. "The administration funneled $1.3 billion last year into the Basic Health Program without a congressional appropriation — a clear violation of the law."

HHS Secretary Sylvia Mathews Burwell has testified that the program, which subsidizes healthcare for people whose income level straddles the border between Medicaid coverage and the plans available on the Obamacare exchanges, can be funded through tax credits.

But Republican lawmakers say HHS is spending taxpayer money without congressional approval. "I would argue this is unconstitutional and it's illegal," Rep. Peter Roskam, R-Ill., one of the lead investigators of the program, said last June. "There is a real problem here when administrations spend money that's not appropriated and they're just doing it with impunity."

The subpoenas could signal the return of the healthcare law to a national political debate that has shifted towards immigration and national security over the last year.

HHS gave Congress "heavily redacted" documents in November, according to the lawmakers. Last month, department staff told House investigators that they wouldn't produce any more documents.

"Our good faith attempts to learn basic, and targeted facts have been met with resistance every step of the way," Brady and Upton said. "The American public deserves answers now."