A senior Trump administration official told Foreign Policy that “disclosing things like this was new for them.”

For years, Iran was prohibited by numerous U.N. Security Council resolutions from developing, testing or transferring ballistic missiles on the grounds that they could be used to advance what the U.S. and other Western powers feared was a clandestine nuclear weapons program.

But under the terms of a landmark 2015 nuclear pact, the missile prohibition was replaced by an appeal to Tehran to voluntarily halt the development of ballistic missiles that can be used to deliver a nuclear payload. Iran, which has long denied it intends to produce nuclear weapons, has taken that as a green light to advance its missile program.

The Trump administration has been struggling for months to identify ways to apply greater diplomatic pressure on Tehran, which is supporting a network of Shiite militias in Lebanon, Iraq, Syria, and Yemen, to expand its influence and power in the region. But the administration’s effort to rally international support for its policies on Iran has been stymied by Trump’s threats to abandon the Iran nuclear deal.

A chief goal of U.S. policymakers is to convince U.N. experts to conclude Iran violated the arms embargo in an upcoming report in December to the Security Council by U.N. Secretary-General António Guterres. That, they hope, could pave the way for the United States to secure support from key allies, particularly European powers that are deepening economic ties with Tehran, for a tougher stance with Iran.

U.S. officials say they are convinced that Iran can be linked to the Nov. 4 missile strike, which Saudi Arabia claims it intercepted on approach to the King Khalid International Airport near the Saudi capital of Riyadh. Less than a week after the November airport attack, the U.S. Air Force’s top officer at Central Command, Lt. Gen. Jeffrey L. Harrigian, said the missile bore “Iranian markings.”

Nikki Haley, the U.S. ambassador to the United Nations, said earlier this month that the Houthis also fired an Iranian Qiam 1 short-range missile in a July attack on an oil refinery in Yanbu.

Yemen’s military has retained prewar stockpiles of SCUD-B and Hwasong-6 missiles, some of which have been modified, according to the U.N. report.

The U.N. panel raised the prospect of an Iranian role in Yemen’s missile program, writing that it could not rule out the possibility that Yemen may be receiving technical advice from “foreign missile specialists,” or that its missiles may have been altered to extend their range.

It also cited the discovery of a shipment of industrial equipment, including “corrosion resistant storage tanks virtually identical to those used to support SCUD SRBM [short-range ballistic missile] operations.” The equipment, according to the panel, “almost certainly originated in the Islamic Republic of Iran.”

The Houthis have claimed their attacks on the airport and refinery employed a missile called a Burqan 2H, which is akin to a SCUD C missile. But the range of the typical SCUD C only has roughly a 700-kilometer (435-mile) range and yet the targets hit in the recent attacks were more than 200 kilometers beyond that distance, U.S. officials said.

The senior administration official told Foreign Policy that missile debris recovered after the airport and refinery attacks resembled an Iranian short-range ballistic missile, the Qiam. The official showed unclassified photos to FP that noted the missile debris had the same exhaust ports and other design features that matched the Iranian weapon. Scud missiles have fins, but tail sections of the missiles recovered in Saudi Arabia had no fins — another similarity to the Qiam design.

“It doesn’t physically resemble the SCUD C,” said the official, who spoke on condition of anonymity.

The photos also showed a fan jet recovered from the missile debris bearing the logo of an Iranian defense company, Shahid Bagheri Industries, the official said.

The panel of experts, however, had said in its report earlier this month that supporting evidence shared with them was insufficient to prove Iranian involvement. “The supporting evidence provided in these briefings is far below that required to attribute this attack to a Qiam-1 SRBM,” the panel wrote in their report.

In addition to the recovered debris, the United States has “other” unspecified intelligence to help make the case that the missiles were Iran-made, and plans to share some of that intelligence with the U.N. team, the U.S. official said.

“The Iranians have consistently said there’s no Iranians in Yemen, that these are really just clever Houthis that paid attention in science class,” the official.

The evidence from the recent missile attacks proved otherwise, the official said, and the declassification effort presented “an opportunity to show European allies what the Iranians are doing.”

Apart from the ballistic missile attacks, Trump administration officials say Iran is allegedly behind improvements in Houthi surface-to-surface missiles as well as other weapons, making the projectiles more precise.

Saudi Arabia’s foreign ministry said investigators studying the intercepted missile uncovered evidence proving “the role of Iranian regime in manufacturing them.”

There is “ample proof, going back more than a year, or Iran providing arms to the Houthis,” said one former State Department foreign service officer, who like other U.S. officials speaking on the issue, ask that their name not be used. “There is no credibility to the Houthis’ claim they made the missiles themselves.”

Experts said they welcomed the Trump administration decision to pursue its case for Iranian involvement to the United Nations, saying it would make it easier to secure European support for a tougher line on Iran.

The senior U.S. official said there were other clues suggesting Iran’s hand in the Yemen conflict. A Houthi boat captured off the Yemeni coast by the Emiratis, who are part of the Saudi-led coalition waging war against the Houthi rebels in Yemen, had a computer system that seemed to be manufactured in Iran.

The boat’s control system included circuit boards that had markings for FHM Electronics, an Iranian joint-stock company in Tehran, the official said. The boat’s autopilot software resembled software advertised in the Iranian firm’s catalog, and contained various GPS coordinates including one corresponding with the location of a known Iranian Revolutionary Guard research organization in Tehran.

The diplomatic initiative is playing out against a backdrop of rising criticism in Congressand abroad of the U.S.-backed Saudi coalition. Two days after Nov. 4 strike, the coalition announced it had shut all land, air, and sea ports in Yemen to halt the smuggling of arms into Yemen. But the blockade, which triggered a famine warning from the U.N. early warning system, prevented all food, medicine, fuel, and other goods from entering the Houthi controlled port of Hodeida, and has sparked international condemnation.

“#Yemen risks starving to death,” the International Committee of the Red Cross warned on Twitter Tuesday. “70% of the population is reliant on humanitarian aid — that’s not getting in.”

The United States, United Kingdom, and other Saudi allies have “only weeks to avoid being complicit in a famine of biblical proportions,” Jan Egeland,  a senior U.N. advisor on humanitarian issue and the secretary general of the Norwegian Refugee Council, tweeted. “Lift the blockade now.”