As federal investigators continue to look into Russian interference in the 2016 presidential election, prosecutors are also probing an Obama-era sale of a uranium mining company.

Attorney General Jeff Sessions last year directed federal prosecutors to look into the sale of Uranium One to a Russian company – a transaction that President Trump has called the “real Russia story.”

The Hill reported that Russian officials engaged in a “racketeering scheme” to further its energy goals in the U.S. And an FBI informant recently told congressional committees that Russia paid millions to a U.S. lobbying firm in an effort to influence then-Secretary of State Hillary Clinton to make sure the deal was successful.

What was the Uranium One deal?

In 2013, Rosatom, backed by the Russian state, acquired a Canadian uranium mining company, now called Uranium One, which has assets in the U.S. Uranium is a key material for making nuclear weapons.

Through the deal, Russia is able to own about 20 percent of U.S. uranium production capacity. However, Colin Chilcoat, an energy affairs specialist who has written extensively about Russia's energy deals, said that the company only extracts about 11 percent of uranium in the U.S.

The deal also “doesn’t allow for that uranium to be exported at all,” Chilcoat told Fox News. “It’s not like it’s leaving the U.S. or somehow finding its way to more insidious players.”

The agreement was approved by nine government agencies with the Committee on Foreign Investment in the United States (CFIUS), an inter-agency group that reviews how certain foreign investments can impact national security. The State Department under Clinton was one of those agencies, though Clinton told WMUR-TV in 2015 that she was not “personally involved” in the agreement.

Why is it controversial?

Some investors reportedly donated millions of dollars to the Clinton Foundation. Former President Bill Clinton also received a $500,000 speaking fee in Russia and reportedly met with Vladimir Putin around the time of the deal, Republicans, who are largely critical of the deal, have said.

The FBI had looked into the agreement and uncovered that some Russian nuclear industry officials were engaged in nefarious dealings, which included extortion, bribery and kickbacks, The Hill reported. Evidence of wrongdoing by Vadim Mikerin, the Russian official overseeing Putin’s nuclear expansion in the U.S. who was eventually sentenced to prison, was discovered by the FBI before the deal was approved, according to The Hill.

Author Peter Schweizer – who wrote about the deal in his 2015 book “Clinton Cash” – told Fox News that there is no evidence that the people involved with approving the agreement knew that the FBI had an ongoing investigation into it.

But White House Press Secretary Sarah Sanders told Fox News “if anyone colluded for a foreign government in [the 2016] election, it was the Clinton campaign [and] the Democrats.”

What did the informant reveal?

Douglas Campbell, the FBI informant, alleged that Moscow paid millions of dollars to a lobbying firm to help Bill Clinton’s charities in order to influence Hillary Clinton, who was then former President Barack Obama’s secretary of state.

Campbell made the claims in a 10-page statement given to the Senate Judiciary Committee, House Intelligence Committee and House Oversight and Government Reform Committee.

Campbell said Russian nuclear officials “told me at various times that they expected APCO to apply a portion of the $3 million annual lobbying fee it was receiving from the Russians to provide in-kind support for the Clinton’s Global Initiative.”

"Your real Russia story is uranium."

— President Donald Trump

“The contract called for four payments of $750,000 over twelve months,” Campbell said in the statement. “APCO was expected to give assistance free of charge to the Clinton Global Initiative as part of their effort to create a favorable environment to ensure the Obama administration made affirmative decisions on everything from Uranium One to the US-Russia Civilian Nuclear Cooperation agreement.”

APCO Worldwide is a global public affairs consulting agency. In a statement to Fox News, APCO said Campbell’s allegations are “false and unfounded.”

“The key issues at stake in this investigation are all about intent and knowledge: was there an intent to influence official business, and, if so, did the recipient take the money in exchange for taking official action,” Jamil Jaffer, a former counsel in the Justice Department and the director of the National Security Law and Policy Program at George Mason University’s Antonin Scalia Law School, told Fox News.

But Jaffer said the credibility of the so-called informant will also come into play.

“Was this a foreign agent or criminal who turned? Was this a private individual the FBI placed inside [the deal]? Was this a government employee? All these factors, plus the level of the informant’s access to relevant information, will make a big difference here,” Jaffer said.

But what does this deal have to do with the Russia investigation?

Multiple congressional committees, as well as the Justice Department, are looking into possible Russian collusion in the 2016 presidential election – and ties between Russians and Trump’s campaign.

"That's your real Russia story. Not a story where they talk about collusion and there was none. It was a hoax. Your real Russia story is uranium," Trump has said.

Robert Mueller, the special counsel leading the probe into alleged Russian interference in the election, was the head of the FBI when it investigated Rosatom officials’ extortion and corruption.

And the investigation was led by then-Assistant FBI Director Andrew McCabe, then-U.S. Attorney Rod Rosenstein, The Hill reported. Rosenstein is now the deputy attorney general; McCabe, until last month, was the deputy director of the FBI.

Mueller's investigators in the Russia probe report to Rosenstein.

The special prosecutors instructed by the Justice Department to investigate “certain issues” pertaining to the Uranium One deal will also report to Rosenstein and Sessions, according to a letter obtained by Fox News.

Congressional committees are looking into whether Mueller informed the Obama administration, particularly those tasked with approving the Uranium One deal, prior to CFIUS approval.

In her attempt to discredit reports of the controversy surrounding the Uranium One deal, Clinton said Trump and “his allies,” are diverting from the investigation.

“The closer the investigation about real Russian ties between Trump associates and real Russians … the more they want to just throw mud on the wall,” she said. “I’m their favorite target, me and President Obama.”

Fox News' Brooke Singman, John Roberts and The Associated Press contributed to this report.