Hillary Clinton has some explaining to do about one of the Clinton Foundation’s biggest benefactors — Ukrainian oligarch Victor Pinchuk. According to Newsweek, the 54-year-old courted the Clintons at least nine times and has been trading with Iran.

In fact, earlier in the year, he was confirmed as the largest individual contributor to the foundation.

Pinchuk owns Interpipe Group, a Cyprus-incorporated maker of seamless pipes for the oil and gas industry. Newsweek points out that both rail and oil and gas sectors are sanctioned by the US, “which specifically prohibits any single invoice to the Iranian petrochemical industry worth more than $1m.”

A copy of a letter seen by Newsweek, is what former Texas Rep. Steve Stockman calls a “body of evidence” that details “exports from Interpipe to Iranian entities” and “may have contravened US sanctions to Iran.”

Pinchuk’s company may be eligible for penalties just by having a U.S. subsidiary in the United States, North American Interpipe Inc. Non-U.S. companies can also be penalized that do not have U.S. based offices. Companies blocked from American markets have difficulty surviving and the government official in charge of maintaining the non-U.S. company list is the secretary of state.

Newsweek says that Pinchuk’s company was trading with Iran between 2009 and 2013—during Clinton’s tenure in the State Department. It should be noted that Pinchuk became very close with the Clintons through Bill Clinton. Pinchuk invited Mr. Clinton to his 50th birthday party at a French ski resort five years ago.

One year later, Pinchuk was a guest at Clinton’s 65th birthday bash as well as a guest at the inauguration of his library in Arkansas. The Clintons have also given remarks at Pinhuk’s Yalta European strategy conference.

Clinton recently removed herself from the board of the foundation.