Two leading House and Senate committees are examining the FBI’s handling of its investigation into Russia’s possible links to Trump campaign associates and the country’s alleged interference with the 2016 presidential election.

The Senate Judiciary Committee, headed by Sen. Chuck Grassley, R-Iowa, is investigating whether the FBI wrongly included political opposition research from Trump’s opponents in its probe, and then paid the author of that controversial report, a former British spy, to work for the FBI on its investigation. The committee’s probe began March 6 with the letter Grassley sent the FBI and was furthered Monday with requests for information from the company that did the opposition research.

“When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics,” Grassley said Monday.

The House Intelligence Committee, headed by Rep. Devin Nunes, R- Calif., is looking into how classified documents containing foreign surveillance transcripts with references to Trump’s transition team were illegally disclosed to the media. The committee’s probe began Jan. 25.

The leaks could have come from the FBI, a source close to the investigation notes, because that agency requested multiple Foreign Intelligence Surveillance Act (FISA) warrants that helped capture some of the foreign surveillance. In addition, sources say, the FBI is not cooperating with the House investigation, unlike the National Security Agency, which has been transparent with the committee. In addition, multiple sources suggest that British intelligence also passed along information to U.S. intelligence agencies.

Meanwhile, the FBI will take full control over the law enforcement investigation into Russia’s interference in the election, Trump’s possible ties to Russia, as well as the leaks, Fox News has learned.

In Grassley’s probe, he is calling into question the FBI’s use of a “controversial and unsubstantiated dossier” compiled by a political opposition research company against then-presidential candidate Trump.

Fusion GPS, a Washington, D.C.-based research and strategic intelligence company, was paid during the campaign by backers of Trump’s Republican and Democrat opponents to perform opposition research, Grassley said. And that company hired former British spy Christopher Steele to write the dossier that was distributed widely to political opponents, the media and the FBI.

The unverified reported was published by the online publication BuzzFeed and included embarrassing allegations that Russian intelligence supposedly could use against Trump.

Most concerning, Grassley said, is that “Fusion GPS and Steele reportedly shared the dossier with the FBI, which then offered to pay Steele to continue his political opposition research on Trump.”

Grassley wants to determine “the extent to which the FBI has relied on the political dossier in its investigation.” The senator also has requested documentation from Fusion GPS as to who hired and paid them, when Steele was hired, how the FBI got involved and whether Fusion GPS was aware of the FBI paying Steele.

Meanwhile, House Intelligence Chairman Nunes said last Wednesday that a source in the intelligence community presented him with “dozens” of reports that were produced from “incidentally collected” communications between members of the Trump transition team and foreign targets. Nunes met with his intelligence source at a Sensitive Compartmented Information Facility (SCIF) in the old executive office building on the White House grounds where they could access the computers without being noticed. They couldn’t go to the source’s agency and use the secured computer network, a source told Fox News, because it would “out” the source.

Nunes said Trump staff members’ identities reportedly were “unmasked” within intelligence agencies through foreign surveillance unrelated to Trump or Russia, and the names were illegally disseminated among intelligence agencies and to the media in what many believe was an effort to embarrass Trump and undermine his presidency.

At least one of those unmasked was former National Security Adviser Michael Flynn, who had information about his communication with the Russian ambassador leaked to press, resulting in a public scandal and his resignation.

Nunes’ committee, like the FBI, has been looking into what actions Russia took against the U.S. during the 2016 election, whether anyone from a political campaign conspired in the activities; whether the communications of officials or associates of any campaign were subject to any kind of improper surveillance; and which intelligence officials leaked classified information that exposed foreign surveillance, conversations between President Trump and other world leaders.

While Nunes refutes Trump’s claims that Obama had him wiretapped during the campaign, Nunes said “…. it's still possible that other surveillance activities were used against President's Trump and his associates.”

An FBI spokesperson said the agency does not have a comment on Grassley’s letter or any additional comments on the House probe.