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A United Nations agency under fire for shipments of computers and other sophisticated equipment to North Korea and Iran has apparently rejected a request by the U.S. State Department to conduct an independent probe into the controversy, drawing a pointed bipartisan rebuke from top lawmakers on Capitol Hill.

In a letter being released Tuesday, the leaders of the House Foreign Affairs Committee complained to World Intellectual Property Organization Director General Francis Gurry about his agency's refusal to cooperate. They accused the agency of locking down key documents while trying to root out the whistle-blowers who alerted others to the scandal -- and then rebuffing the State Department's request for an outside investigation.

"We are outraged by your recent refusal on the basis of 'confidentiality,' of a request by the U.S. Department of State to conduct an independent, external investigation into how and why these transactions happened," wrote Reps. Ileana Ros-Lehtinen, R-Fla., and Howard Berman, D-Calif., chairwoman and ranking member of the committee, respectively. "There is no rational basis for this refusal. ...  On the face of it, the documentary record, coupled with your public statements, shows a shocking and intolerable lack of judgment, together with an inclination to disregard the legitimate concerns of Member States and to retaliate against staff who are simply trying to tell the truth.

"However, if you truly believe that your actions have been entirely proper, then surely you would have nothing to hide and no reason to block the requested independent investigation," they wrote.

The lawmakers continued to call for an independent probe. And they described an agency pledge to have future shipments to Iran and North Korea reviewed by the U.N. Sanctions Committee "not sufficient."

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The House Foreign Affairs Committee had already announced earlier this month it was moving forward with its own investigation into the shipments. In their letter, Ros-Lehtinen and Berman called on Gurry to provide their committee with "unfettered access" to documents.

The U.N.'s World Intellectual Property Organization claims it was just shipping "standard IT equipment" and did not violate sanctions.

But the inquiries, including one by the State Department, raise questions about the ways in which U.N. agencies have managed to sidestep restrictions that the world body expects the rest of the world to obey in halting the spread of sensitive technologies to nuclear-ambitious pariah regimes.

It also calls into question how much U.N. member states know about the activities of agencies they supposedly approve and supervise.

The State Department probe came in the wake of Fox News revelations in April about the actions by WIPO in sending such sensitive equipment to North Korea by a complicated method that seemed designed to bypass U.N. Security Council sanctions against the country.

The shipments took place in late 2011 or early 2012, and were financed through the Beijing offices of the United Nations Development Program (UNDP).

The WIPO actions also violated the sweeping restrictions of the equipment manufacturer, Hewlett-Packard, which forbids any HP equipment from being sent to such regimes.

The U.S. is a member of WIPO, yet apparently knew little or nothing about the controversial delivery of computers and sophisticated services. And within a month, the State Department discovered the problem went beyond North Korea, a spokesman said in response to questions from Fox News.

In the case of Iran, the WIPO computer shipment included 20 Hewlett-Packard Compaq desktop computers, now outmoded in the U.S. but which nonetheless still gave Iran's Industrial Property Office significant computing power. In the case of North Korea, the equipment included more sophisticated computers and data-storage servers.

As was the case in North Korea, WIPO experts made technical visits to Iran in advance of the shipments to scope out the project, help orchestrate financing and payment by the local office of UNDP in Tehran, and OK the deliveries, according to WIPO's documentation.

Click here for WIPO documents.

Fox News' George Russell contributed to this report.