Rep. Devin Nunes, R-Calif., said on Wednesday he is certain that someone on foreign soil was "running operations" against the Trump campaign.

The House Intelligence Committee ranking member was invited onto Fox News to discuss a letter he sent to President Trump with a list of questions to ask British Prime Minister Theresa May about the Steele dossier when he visits the United Kingdom next month.

Host Sean Hannity asked Nunes about the possibility that members of the U.S. intelligence community in the Obama administration "outsourced what would be illegal intelligence gathering" to allies such as the United Kingdom.

"I think we can be sure about one thing and that is that on foreign soil, someone was running operations against Trump campaign people," Nunes said. "Now, we don't know if that was our government, if it was the British government, or if it was some type of firm of some kind that was doing it. But I think it is definitely a fact, Sean, that at least on British soil, possibly on Italian soil, there were activities going on that we need to get to the bottom of."

Nunes has been a leading critic of the FBI's use of British ex-spy Christopher Steele's dossier, filled with unverified claims about Trump's ties to Russia, to obtain Foreign Intelligence Surveillance Act warrants to wiretap onetime Trump campaign adviser Carter Page.

At least three federal investigations into alleged FISA abuse and other matters related to the way the FBI and the Justice Department conducted the Trump-Russia investigation and several top ex-officials, including former CIA Director John Brennan and former FBI Director James Comey, are under increasing scrutiny.

Nunes' letter, obtained by the Washington Examiner, cites a report from the Telegraph published over the weekend that said the leaders of MI5 and MI6, the United Kingdom’s top intelligence agencies, were briefed about Steele’s dossier shortly after the 2016 election, before Trump was told about them.

One subject of Nunes' letter to Trump is Maltese academic Joseph Mifsud, the man who told former Trump campaign adviser George Papadopoulos the Russians had damaging information about Clinton. Papadopoulos later repeated this claim to Australian diplomat Alexander Downer, who informed the U.S. government and prompted the original counterintelligence investigation into Trump's campaign in July 2016.

Although special counsel Robert Mueller's team portrayed Mifsud, London-based professor, as a Russian asset with close ties to Russian President Vladimir Putin in their 448-page report, Nunes says they decided to "cherry pick" information from news reports, leaving out that he is described as a Western intelligence asset. Nunes specifically noted that Mifsud has a lot of ties to U.S., British, and Italian intelligence services.

Nunes told Hannity that he believes Mueller's team, which he dubbed the "Mueller dossier team," knew British intelligence had warned their U.S. counterparts about Steele's credibility.

"I actually believe that Mueller knows that the Mueller dossier team knew that the Brits actually I think did give us some warning. But we can't get that information. I think there are things that Mueller knew about. I think the Brits probably did raise concerns about Steele's credibility to the new administration. But of course, we can't get those documents and it's what we continue to try to get," Nunes said.

The congressman said he hopes U.S. Attorney John Durham, who has been tasked by Attorney General William Barr to review the origins of the Russia investigation, will be able to get the answers he seeks.

The Justice Department and House Intelligence Committee struck a deal on Wednesday to allow the panel access to counterintelligence material underlying Mueller's work.