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Federal officials have serious questions about the credibility of a key witness in congressional investigations of Hillary Clinton’s role in the sale of a uranium-mining company to Russian interests, two sources knowledgeable about the case tell Yahoo News.

The witness, a Florida businessman and former FBI informant named William Douglas Campbell, was considered so unreliable that prosecutors dropped him as a witness in an unrelated case involving Russian uranium sales, according to the sources.

The investigations by two House subcommittees, apparently spurred by a tweet from President Trump last month, focus on allegations about Clinton’s role in approving the controversial sale of Uranium One, a company that owned American uranium mines, to Rosatom, the Russian atomic energy company, in 2010. Campbell has suggested he can prove that approval of the sale was a quid pro quo for donations by Russian-connected parties to the Clinton Foundation.

But there are mounting questions about Campbell’s credibility in light of his track record as an informant in a separate FBI investigation into a Russian businessman named Vadim Mikerin, who was in charge of U.S. operations for Tenex — a separate unit of Rosatom that was not involved in the Uranium One purchase.

Court records and interviews with the sources who are familiar with the case indicate that Campbell provided key information to the FBI about a scheme orchestrated by Mikerin to collect kickbacks from American companies doing business with Tenex. The bureau enlisted Campbell as an undercover informant who wore a wire in his conversations with Russian officials.

But he proved a “disaster” as a potential witness in the case when doubts arose about his descriptions of some events that could not be documented, one of the sources said. As a result, prosecutors dropped extortion charges against Mikerin that relied on Campbell’s testimony. “There was no question that [Campbell’s] credibility was such that [the prosecutors] had to restructure the case,” the source said. “He got cut out of the case entirely.”

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Mikerin pleaded guilty to the remaining money laundering charges in August 2015 and was sentenced to four years in prison, which he is still serving.

“This is a smear job,” Victoria Toensing, Campbell’s lawyer, told Yahoo News when asked about the doubts law enforcement officials had about her client’s credibility. “That was not the reason” prosecutors removed him as a witness against Mikerin, she said. Instead, it was because “the Obama administration didn’t want to bring a big extortion case against Russia” and risk testimony from Campbell that would have undermined the Uranium One sale and undercut the president’s efforts to “reset” relations with Russia. So instead, “they covered it up,” she said.

The questions about Campbell and his purported knowledge about the Uranium One sale have become a political lightning rod in recent weeks, with Democrats charging they are only being raised now to distract attention from special counsel Robert Mueller’s investigation into the Trump campaign and its ties to the Kremlin.

Top Republicans initiated the Uranium One probes after President Trump tweeted on Oct. 19 that the sale to Russia “with Clinton help and Obama administration knowledge is the biggest story that Fake Media doesn’t want to follow!” Two House panels — subcommittees of the House Oversight and Government Reform Committee and the House Intelligence Committee — have begun investigations. House Republican leaders are also calling for the appointment of a Justice Department special counsel to conduct a criminal probe of the Uranium One deal.

In announcing one of the congressional inquiries three days after Trump’s tweet, Rep. Ron DeSantis, chairman of the House Oversight and Government Reform National Security Subcommittee, said he had “explosive” information from a “confidential informant” who he said “would be able to link” the Obama administration’s approval of the sale of Uranium One to Russia to “millions of dollars” in donations to the Clinton Foundation. The sale of Uranium One was unanimously approved by the Committee on Foreign Investment in the United States (CFIUS), an interagency panel of nine Cabinet members on which Clinton, who was then secretary of state, sat.

Campbell, who is expected to testify behind closed doors to DeSantis’s panel, repeated that claim in an interview with Reuters this week for a story that named him publicly for the first time. He told the news agency that he had worked for the Justice Department “undercover for several years,” and that he had “documentation” that could show a relationship between approval of the Uranium One sale by the Obama administration and “political influence.” (Toensing, Campbell’s lawyer, said her client was “dazed” at the time of the interview because the reporter caught up while he was recovering from chemotherapy treatment for cancer. She said “there are documents” in Campbell’s possession, but declined to specify what they were.)

But sources with detailed knowledge of his earlier role are skeptical. The sources confirm that Campbell approached the FBI with information about Mikerin’s role in a kickback scheme involving American companies doing business with the Tenex unit of Rosatom. Campbell was hired as a consultant to Tenex and was also paid by Cassidy Associates, a major Washington lobbying firm, that had been hired by Tenex to promote its business in the U.S.

The investigation that followed was a significant one — far more so than was publicly revealed at the time. The FBI was able to trace $2.1 million in bribes paid to Mikerin and others and wired to offshore shell companies in Cyprus, Latvia and Switzerland. U.S. intelligence officials suspected — but were never able to conclusively prove — that these payments ultimately benefited entities connected to Vladimir Putin.

But Campbell’s importance to the Mikerin investigation faded after prosecutors discovered discrepancies in his account of his dealings with Mikerin and other Russian figures, sources said. Investigators also were unable to verify some of his more explosive allegations, including claims that he was threatened with violence if he did not participate in the scheme by helping to arrange kickbacks in exchange for contracts with Tenex. The doubts were serious enough that the U.S. attorney’s office in Maryland — which was prosecuting Mikerin under the direction of then U.S. Attorney Rod Rosenstein, now the deputy attorney general — felt compelled to alert defense lawyers to them. There were also other questions about his background, including two DUI convictions in 2009 and 2012.

Eventually, fearing Campbell would not withstand scrutiny in open trial, prosecutors dropped him as a potential witness and scrapped the charges against Mikerin that would have relied on his testimony.

Those problems arose again last year when Campbell filed a lawsuit against Mikerin in federal court in Baltimore, accusing him of racketeering and seeking damages. At the time, Mikerin was already serving his sentence at a federal prison in North Carolina. Because legal proceedings were still pending against another defendant in the case, Justice Department prosecutors wrote to Campbell’s then lawyer, accusing him of violating the terms of his agreement with the government. Prosecutors threatened to reveal derogatory information to the judge about Campbell’s dealings with the government If he pursued the civil case,. Campbell later dropped the suit.

Toensing, Campbell’s lawyer, said that Campbell would be able to tell Congress that discussions about the Uranium One sale were raised by Russian officials during his dealings with them. But sources who spoke to Yahoo News said that during the course of hours of interrogations by the FBI and Justice Department lawyers, Campbell never mentioned any connection between Mikerin’s dealings and the sale of Uranium One. Nor did he ever volunteer that he had documents or information relating to the Uranium One sale. The issue, one of the sources said, never came up.

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