A document obtained this week by the Associated Press indicates that Iran—operating under a secret agreement with a United Nations agency—will be allowed to use its own inspectors to investigate a site it has been accused of using to develop nuclear weapons.

The revelation caught the immediate attention of Republican lawmakers who have been severely critical of a broader agreement signed by the Obama administration to limit Iran’s future nuclear programs. Those critics have complained that it is not wise to trust the Iranians to maintain the bargain of unfettered inspections of its nuclear facilities. The newly-disclosed side agreement, which allows for investigation of Iran’s Parchin nuclear site by the U.N.’s International Atomic Energy Agency (IAEA), is reportedly linked to persistent allegations that Iran continues work to develop atomic weapons.

“This side agreement shows that true verification [is] a sham, and it begs the question of what else the administration is keeping from Congress,” said House Majority Leader Kevin McCarthy (CA-23). McCarthy further complained that Congress learned of the IAEA deal not from the administration, but from the press.

The Parchin agreement was reportedly completed between the IAEA and Iran; the United States and five other world powers were party to it, but were briefed by the IAEA and endorsed it as part of the larger package.

Evidence of the inspectors’ concession, as outlined in the document, may increase pressure from U.S. Congressional opponents before a Senate vote of approval or disapproval on the overall agreement takes place early next month. If the resolution passes and President Barack Obama vetoes it, opponents would need a two-thirds majority vote to override it. Senate Majority Leader Mitch McConnell has suggested those who oppose the pact will likely lose a veto fight.