WASHINGTON — FBI Director James Comey Monday confirmed for the first time publicly that federal authorities were investigating the Russian government's interference in the 2016 elections, including communications between associates of President Trump and Russian officials.

Comey, appearing before the House Intelligence Committee along with National Security Agency Director Mike Rogers, said there was no time frame for the completion of the inquiry.

Comey and Rogers both said they knew of no evidence that any actual votes were changed as a result of Russian interference.

Although Trump, as recently as Friday, has continued to defend the extraordinary allegations that the Obama administration wiretapped Trump Tower in advance of the 2016 elections, House and Senate leaders briefed by the FBI director in recent days have flatly denied such surveillance existed in bipartisan joint statements, leaving the White House virtually alone in asserting such claims.

“Let me be clear,'' House Intelligence Chairman Devin Nunes, R-Calif., said Monday, "we know there was not a wiretap on Trump Tower. However, it's still possible that other surveillance activities were used against President Trump and his associates.''

Nunes also said Monday that there is "no evidence to date that officials from any campaign conspired with Russian agents."

California Rep. Adam Schiff, the House committee's ranking Democrat, called Trump's claims ''slanderous,'' adding that "we do not yet know whether the Russians had the help of U.S. citizens, including people associated with the Trump campaign.''

Schiff said that the ongoing effort determing the scope of Russian interference in the U.S. election system represented the most important challenge for U.S. intelligence.

"The stakes are nothing less than the future of liberal democracy,'' Schiff said.

The wiretap accusation has not only shadowed the White House for the past three weeks, it also has triggered diplomatic row with a key ally, as Trump and aides cited a discredited report by Fox News commentator Andrew Napolitano that Obama asked a British intelligence agency to tap Trump. The British government objected and the Trump administration pledged not to use the claim again. Asked about the flap, Trump said: "That was a statement made by a very talented lawyer on Fox, and so you shouldn't be talking to me, you should be talking to Fox. OK?"

Sen. Susan Collins of Maine, one of several Republicans who have questioned the president's allegations, said Sunday on NBC's Meet The Press that "I don't know the basis for President Trump's assertion," and "I do believe he owes us that explanation."

The high-stakes House hearing also is looking into efforts by Russia to influence last year's election by hacking Democratic officials close to nominee Hillary Clinton, the subject of an ongoing investigation by the FBI. During an appearance on Fox News, House Intelligence Chairman Devin Nunes, R-Calif., said he has seen no evidence of collusion between Trump associates and the Russians during the election.

The committee hearing comes a little more than two weeks after Trump leveled his accusations against Obama in an early Saturday morning tweetstorm. One tweet said: "Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"

The charges brought furious denials from Obama aides — who pointed out that the law forbids presidents from ordering wiretaps — to a diplomatic flap with the United Kingdom, which protested claims by Trump allies that the British may have been involved in the wiretapping.

Nunes said Monday's hearing would also look into the possibly illegal leaking of national security information since Trump's election in November. The Intelligence Committee chairman cited the case of former national security adviser Michael Flynn, dismissed by Trump for mischaracterizing his discussions with the Russian ambassador to the United States.

Trump and aides have also denied any connection to Russians who sought to hack Democratic officials during last year's election and said opponents are leaking derogatory information against them as part of a "witch hunt" to undermine the presidency.

For Comey, Monday's hearing represents another unusually high-profile role for the FBI director. Comey was harshly criticized by Republicans for the public announcement in July that he was not recommending criminal charges against Clinton for her use of a private email server while secretary of State. He drew the wrath of Democrats in October for announcing that the bureau was reopening its email review, 11 days before the presidential election.

Since Trump made the disputed claims, he and aides have sought to redefine the terms of his accusations. While Trump used the term "wire tapping," he and his aides say that now refers to "surveillance" in general.

A number of Republicans have expressed skepticism about Trump's tweets on Obama. House Speaker Paul Ryan, R-Wis., told Fox News Sunday that he has seen no evidence of wiretapping Trump Tower and said he is leaving it to committees to investigate.

"I want to get on with passing our agenda," Ryan said.