An undercover informant who helped the FBI prove Russia's uranium industry engaged in criminal activity says congressional Democrats' recent attacks on his credibility were insulting to his medical disability and ignored the fact that agents recently asked him for fresh testimony about former President Clinton and Hillary Clinton.

In his first on-camera interview, William Douglas Campbell told The Hill he was interviewed for about five hours in December by FBI agents from Little Rock, Ark., who were investigating whether donations to the Clintons's charitable empire were used to influence U.S. nuclear policy during the Obama years.

Campbell said he was asked specifically about whether donations to the Clintons charitable efforts were used to influence U.S. nuclear policy during the Obama year, and that agents questioned him extensively about claims the Russians made to him that they had routed millions of dollars to an American lobbying firm in 2010 and 2011 with the expectation it would be used to help President Clinton's charitable global initiative while major uranium decisions were pending before Hillary Clinton's State Department.

"They were looking into the Clintons, and the information that I provided to them about the Clintons and about what was said and confirmed by Russian leadership seemed to be very important to them," Campbell said, appearing in shadow during the interview to protect his identity.

Spokesmen for the Clinton Foundation and Hillary Clinton did not return calls Thursday seeking comment. The Clintons have long denied donations influenced any policies.

Campbell worked as an FBI undercover informant from 2008 through 2014 inside Russia's nuclear industry, helping to uncover a bribery, kickback, money laundering and extortion scheme that sent several Russian and U.S. executives to prison.

He was summoned for a closed-door congressional interview last month by Republicans, who believe the criminal wrongdoing Campbell uncovered should have stopped the Obama administration from approving the sale of the Uranium One mining firm and billions of dollars in U.S. nuclear fuel contracts to Russia. House Democrats issued a blistering memo attacking Campbell's credibility, saying he couldn't identify specific crimes committed by the Clintons and suffered from memory lapses that required him to rely on written notes.

Campbell dismissed the Democrats' attacks as partisan.

"I am not a Republican. I am not a Democrat. I'm not an independent. I am a damn American," he said. "I'd like to remind those Democratic staff members who wrote that interview summary that none of 'em have ever worked undercover as a confidential informant ... and put themselves in clear and present danger with Russian criminals who are breaking U.S. law."

Campbell said the Democrats' criticism smacked of hypocrisy, because they knew in advance he suffered memory problems from a brain tumor.

"I can remember them running on an American disabilities [platform] wheeling people in wheelchairs and people that were having issues like I was having in and talking about how sacred that was to the Democratic Party. And hell, they're mocking me?" he said. "It's not right."

Spokesman for Democrats on the House Intelligence, Judiciary and Oversight and Government Reform committees, who wrote the Campbell memo, did not respond to a request for comment Thursday.

Campbell also disputed allegations by anonymous Justice officials and Democrats that while undercover he may have engaged in illegal payments with the Russians without approval. He said Moscow asked him to pay $25,000 in 2010 to hire a consultant to train him on nuclear issues and that his FBI handlers "sanctioned and were aware that I was transferring those monies." When the Russians didn't provide the consulting and asked for more money, the agents recognized it was a kickback scheme and authorized him to keep making payments so they could make a criminal case, he said.

He dismissed suggestions he lacked credibility, noting the FBI recently asked him for fresh information and paid him a $51,000 reward in 2016.

"I was embraced and told what a good job I had done," he said.