A former top Clinton administration diplomat who used his political sway to garner support for the Iran nuclear deal apparently was being bankrolled the entire time by Boeing -- which is set to make billions off a jet deal with Tehran now that sanctions have been lifted.

Thomas Pickering, who also served as co-chairman of the board examining the Benghazi attack response, publicly pushed for the nuclear deal before its approval last year. He did so by penning op-eds, writing to high-level officials and even testifying before Congress.

With the deal in place, Boeing has since moved forward on a $25 billion deal with Iran Air made possible by the nuclear agreement.

While Pickering never denied being on Boeing’s payroll during the talks, he didn’t regularly disclose it either, according to a new report in The Daily Beast. And that’s the problem, transparency advocates say.

“In Pickering’s case, he has a direct connection to Boeing, which I think should be disclosed,” Neil Gordon, an investigator for the Project on Government Oversight, told The Daily Beast. “I think it’s necessary for the public debate. It’s necessary for the public to fully realize the participants’ financial interests. Some of them might have a direct financial stake in a particular outcome.”

Pickering was a former top State Department official in the Bill Clinton administration, and before that ambassador to Russia. He also served as ambassador to the United Nations, Israel and elsewhere in prior administrations.

When Pickering testified before the House Armed Services Committee on June 16, 2014, the biography provided to committee members touted his military and government services but did not list his business ties.

Pickering also sent a July 7, 2015 letter to lawmakers urging them to back the nuclear deal but reportedly did not make his association with Boeing known. The letter was cited by the media, lawmakers and the White House in the push to sell the nuclear deal to the public.

In op-eds for The Washington Post and Tablet, he also made the case for the deal but again did not disclose his ties.

He confirmed to The Daily Beast that he was a Boeing employee from 2001 to 2006 (which was more widely known) and later worked as a "direct consultant" from 2006 to 2015.

Earlier this month, Boeing reached a tentative agreement to sell passenger planes to Iran’s state-run carrier, Iran Air. The deal is the first major business venture after sanctions were eased against Tehran last year and is seen by many as a groundbreaking test for other American companies looking to profit from Iran’s untapped economy.

The deal is still in its early stage and will likely face scrutiny from U.S. trade regulators and lawmakers.

“It’s tragic to watch such an iconic American company make such a terribly short-sighted decision,” Rep. Peter Roskam, R-Ill., told FoxNews.com in a statement. “If Boeing goes through with this deal, the company will forever be associated with Iran’s chief export: radical Islamic terrorism. The U.S. Congress will have much to say about this agreement in the coming days.”

Roskam and Rep. Jeb Hensarling, R-Texas, sent a letter to Boeing CEO Dennis Muilenburg last week raising concerns about Tehran’s history of using commercial planes to support "hostile actors."

“We strongly oppose the potential sale of military-fungible products to terrorism’s central supplier. American companies should not be complicit in weaponizing the Iranian Regime,” the lawmakers wrote.

Boeing wrote back saying it would follow the lead of the U.S. government with regards to working with Iran Air and that “any and all contracts with them will be contingent upon continued approval.”

“And as we have stated repeatedly, should the U.S. Government reinstate sanctions against the sale of commercial passenger airplanes to Iranian airlines, we will cease all sales and delivery activities as required by U.S. law,” Tim Keating, Boeing senior vice president, wrote.

Five years ago, the Obama administration slapped sanctions on Iran Air, claiming the company used passenger and cargo planes to transport rockets and missiles to places such as Syria, sometimes disguised as medicine or spare parts. In other cases, members of Iran’s Revolutionary Guard Corps took control of flights carrying sensitive cargo.

Although U.S. officials never said such conduct ended, the administration used a technicality to drop those sanctions as part of last year's seven-nation nuclear deal. The agreement also allowed the Treasury Department to license American firms to do business in Iran's civilian aviation sector. The changes enable Boeing to sell up to 100 aircraft to Iran Air, by far the most lucrative business transaction between the U.S. and Iran since the 1979 Islamic Revolution and U.S. Embassy hostage crisis.

State Department spokesman John Kirby said the sale and any possible future deals depend on Iran's good behavior.

The U.S. could revoke the license for the deal if planes, parts or services are "used for purposes other than exclusively civil aviation end-use" or if aircraft are transferred to individuals or companies on a U.S. terrorism blacklist, Kirby said.

Any suggestion "that we would or will turn a blind eye to Iran's state sponsorship of terrorism or their terrorist-supporting activities is completely without merit," Kirby said.

The details of the arrangement between Boeing and Iran Air aren't entirely clear. Iran's Transportation Minister Abbas Akhoundi said it could match the $25 billion package between the Islamic Republic and Boeing's European rival, Airbus. Iran Air has stated its interest in purchasing new Boeing 737s -- single aisle jets that typically fly up to five hours. It also wants 777s -- larger planes that can carry passengers for 12 hours or more.

But if Iran Air continues supporting Iranian military or Revolutionary Guard operations, it would put the Obama administration or any successor in a bind.

Revoking the license and suspending future plane transfers risks angering the Iranians, who've already complained about not receiving sufficient benefit for their nuclear concessions. It also could mean billions in lost revenue for a large American company with more than 130,000 employees in the United States.

FoxNews.com's Adam Shaw and The Associated Press contributed to this report.