WASHINGTON — Longtime Trump adviser Roger Stone says he believes his contacts with a Russian-linked hacker who took credit for breaching the Democratic National Committee may have been obtained through a FISA warrant, which allows the government to collect the communications of individuals suspected of being agents of a foreign power.

Stone is among a handful of President Donald Trump’s associates who are under scrutiny for possible links to Russia during the presidential race. In a statement to The Associated Press, Stone said he’s retained two attorneys to explore whether he can compel the government to “either charge me or admit they have no case whatsoever.”

The Republican operative has acknowledged having a Twitter exchange with Guccifer 2.0, a hacker that U.S. officials believe has ties to Russia. Stone said he was “unaware at the time of the brief exchange of allegations that the hacker in question is suspected of being a Russian asset.”

“I have no relationship with the Russian state, Russian Intelligence or any other Russians,” said Stone, who split with Trump’s campaign in August 2015 but remains in touch with the president.

Stone’s messages with Guccifer 2.0 were first reported by the website The Smoking Gun. He said the story contains information that “could only be learned by surveillance of my domain and eavesdropping on my e-mail, phone calls and texts.”

“If these where obtained through a FISA warrant as I believe and the information was leaked to the Smoking Gun, that would constitute a felony,” Stone said. He also suggested, without evidence, the government had Trump under surveillance during the presidential campaign.

Trump has accused former President Barack Obama of wiretapping the New York skyscraper where he lived and ran his campaign, though he has provided no evidence to support his explosive allegation. On Monday, the White House started softening Trump’s claims, with spokesman Sean Spicer saying the president was referring to general surveillance that may have been approved by the Obama administration.

The FBI, as well as House and Senate committees, are investigating Russia’s interference in the 2016 election and possible contacts between Trump associates and Russians. Trump has asked the congressional committees also to investigate his wiretapping allegations, though the Justice Department missed a Monday deadline to provide evidence to the House committee.

Sen. Lindsey Graham, R-S.C., says he’s waiting to hear from FBI Director James Comey whether a warrant was issued that would have allowed the Obama administration to tap Trump’s phones during the campaign. Graham says he asked Comey to answer by Wednesday and also to say whether the FBI is investigating Russian interference in the 2016 campaign.

If Comey doesn’t comply, Graham says Congress “is going to flex its muscle.”

“We’ll issue a subpoena to get the information, we’ll hold up the deputy attorney general’s nomination until Congress is provided with the information to finally clear the air as to whether there was ever a warrant issued against the Trump campaign,” Graham, who heads the Judiciary Committee’s crime and terrorism subcommittee, said Wednesday on NBC’s “Today” show.

Republican Sen. John McCain has singled out Stone as a Trump associate who should answer questions about his ties to Russia. The Arizona lawmaker suggested in an interview Sunday with CNN that Stone may have ties to ousted Ukrainian President Viktor Yanukovych.

Stone denied any connection to Yanukovych or his political party. Stone said he did work for a “small pro-Western splinter party” of Ukrainian politician Volydmyr Litwin, calling the effort to help win seats in parliament a “low budget operation.”

Stone said he is willing to testify in any Senate hearings that occur “in public and not behind closed doors.” He’s retained former U.S. Attorney Kendall Coffey and South Florida lawyer Grant Smith to represent him.

Last year, Coffey represented Trump campaign manager Corey Lewandowski when he was charged with simple battery after an altercation with a female reporter. The prosecutor later dropped the charges.