Vienna, Austria – Iran, in an unusual arrangement, will be allowed to use its own experts to inspect a site it allegedly used to develop nuclear arms under a secret agreement with the U.N. agency that normally carries out such work, according to a document seen by The Associated Press. The revelation is sure to roil American and Israeli critics of the main Iran deal signed by the U.S., Iran and five world powers in July. Those critics have complained that the deal is built on trust of the Iranians, a claim the U.S. has denied.

The investigation of the Parchin nuclear site by the International Atomic Energy Agency is linked to a broader probe of allegations that Iran has worked on atomic weapons. That investigation is part of the overarching nuclear deal. The Parchin deal is a separate, side agreement worked out between the IAEA and Iran. The United States and the five other world powers that signed the Iran nuclear deal were not party to this agreement but were briefed on it by the IAEA and endorsed it as part of the larger package.

Without divulging its contents, the Obama administration has described the document as nothing more than a routine technical agreement between Iran and the U.N.’s International Atomic Energy Agency on the particulars of inspecting the site. During a hearing on Capitol Hill July 23, Sen. Bob Menendez, D-NJ, and Sen. James Hirsch, R-ID, raised the issue of how Parchin would be inspected. Kerry replied that the Parchin inspection was “a classified component” of the deal and wouldn’t go into specifics.

Any IAEA member country must give the agency some insight into its nuclear program. Some countries are required to do no more than give a yearly accounting of the nuclear material they possess. But nations – like Iran – suspected of possible proliferation are under greater scrutiny that can include stringent operations. But the agreement diverges from normal inspection procedures between the IAEA and a member country by essentially ceding the agency’s investigative authority to Iran. It allows Tehran to employ its own experts and equipment in the search for evidence for activities that it has consistently denied – develop nuclear weapons.

Evidence of that concession, as outlined in the document, is sure to increase pressure from U.S. congressional opponents as they review the July 14 Iran nuclear deal and vote on a resolution of disapproval in early September. If the resolution passed and President Barack Obama vetoed it, opponents would need a two-thirds majority to override it. Even Senate Majority Leader Mitch McConnell, a Republican, has suggested opponents will likely lose.