Former Trump campaign adviser Roger Stone told House Intelligence Committee members Tuesday that allegations that he or anyone else in Trump's circle sought to collude with Russian operatives to win the 2016 election were "false."

But Stone refused to answer questions about the intermediaries through whom he made contact with WikiLeaks, prompting committee Democrats to threaten future subpoenas if he is not more forthcoming with information.

Stone spoke with House investigators for almost three hours behind closed doors. He said he mainly answered questions about his contacts with a hacker believed to be behind breaches of the Democratic National Committee's and Hillary Clinton campaign chairman John Podesta's email accounts, and with the founder of WikiLeaks, which released much of the hacked information.

He told reporters afterward that he considered the committee interview "an opportunity to correct a number of things that members of the committee had said about me."

But he declined to answer members' questions about the identity of his intermediary with WikiLeaks' Julian Assange, explaining that the person "is a journalist and our conversation was off the record."

Former Trump campaign adviser Roger Stone speaks to reporters after a closed-door hearing before the House Intelligence Committee on Sept. 26. (Mark Wilson/Getty Images)

Stone has long argued that the collusion allegations against the Trump campaign are unfounded and that there is no actual proof that Russia was the force behind alleged hacks and other efforts to meddle in the 2016 presidential election. He has repeated this argument despite consensus in the intelligence community that the Kremlin tried to sway the election to favor Trump — arguing instead that consensus shows that the intelligence agencies have been "politicized."

Still, several of Stone's campaign-time interactions with suspected Russian intermediaries have raised public suspicion about what role he could have played in establishing alleged ties between the Trump campaign and the Kremlin.

In an opening statement to the committee, Stone sharply denied having any Russian clients or contacts with Russians during the campaign. He also denied that he had any foreknowledge of WikiLeaks' plans to release Podesta's emails, despite tweeting just days before that Podesta's "time in the barrel" was coming.

Stone also stressed that he never coordinated with the hacker Guccifer 2.0 before questioning last year not only the Russians' alleged role in the hacks, but also whether the DNC had ever been hacked. Stone included images of a private conversation he had with Guccifer 2.0 via Twitter — to back his claim — as an addendum to his opening statement. And Stone protested to reporters Tuesday that collusion would have been impossible "unless I owned a time machine, which I do not."

Stone said he had been "fairly treated" by the committee — but described "some clashes between Republicans and Democrats about the appropriateness" of certain questions he was being asked.

He recalled that at one point, Rep. Trey Gowdy (R-S.C.), who is helping to run the panel's Russia probe, asked Stone whether he had any knowledge of Russian collusion, which he denied having.

"Mr. Schiff said, 'Well, we could ask the same question of Vladi­mir Putin,' and Mr. Gowdy took exception to that," Stone added, referring to the panel's ranking Democrat, Rep. Adam B. Schiff (Calif.).

Stone went after Schiff at several points in his prepared opening statement to the committee, which he shared with The Washington Post on Monday night.

Schiff responded to those direct hits Tuesday morning, noting: "It's consistent with his motto: deny, deny, deny."

Stone told reporters Tuesday that the House Intelligence Committee is the only investigating body currently asking him questions.

He said that he has not heard from the office of special counsel Robert S. Mueller III or the FBI and that he has "only had initial contact from" the Senate Intelligence Committee, which asked him to preserve records.

Tom Hamburger contributed to this report.