The top U.S. intelligence official said on Thursday he was “even more resolute” in his belief that Russia staged cyber attacks on Democrats during the 2016 election campaign, rebuking persistent skepticism from Republican President-elect Donald Trump about whether Moscow was involved.

Director of National Intelligence James Clapper said he had a very high level of confidence that Russia hacked Democratic Party institutions and campaign staff email, and disseminated propaganda and fake news aimed at the Nov. 8 election.

“Our assessment now is even more resolute than it was” on Oct. 7 when the government first publicly accused Russia, Clapper told a hearing of the Senate Armed Services Committee. He said a motive for the attack would be made public next week.

Although Trump called himself a “big fan” of intelligence agencies, he is heading for a conflict over the issue because he has cast doubt on their assessments that Russia targeted the election and the campaign of his opponent, former secretary of state Hillary Clinton.

“There’s a difference between healthy skepticism and disparagement,” Clapper said. Vice President-elect Mike Pence has used the expression “healthy skepticism” to defend Trump’s criticism of intelligence agency findings.

Lawmakers from both parties are wary of Moscow and distrust Trump’s praise of Russian President Vladimir Putin and efforts to heal the rift between the United States and Russia.

Thursday’s hearing, overseen by Republican Senator John McCain, one of the most vociferous Russia critics in Congress, was the first in a promised series of hearings into allegations that Russia tried to disrupt or influence the U.S. campaign, one of the most bitter in recent history.

“Every American should be alarmed by Russia’s attacks on our nation. There is no national security interest more vital to the United States of America than the ability to hold free and fair elections without foreign interference,” McCain said.

Clapper said the hacking did not change any vote tallies.

Trump will be briefed by intelligence agency chiefs on Friday on hacks that targeted the Democratic Party before the election, which the New York businessman surprisingly won.

“I don’t think we’ve ever encountered a more aggressive or direct campaign to interfere in our election process than we’ve seen in this case,” said Clapper, who leaves when Trump becomes president on Jan. 20. Clapper stopped short of declaring Russia’s actions “an act of war,” saying that determination was beyond the scope of his office.

MOSCOW DENIES HACKING

Clapper and the two other officials who testified, Admiral Mike Rogers, the director of the National Security Agency, and Marcel Lettre, the under secretary of defense for intelligence, did not say what made intelligence agencies confident Russia was behind the cyber attacks, but that conclusion was also reached by several private cyber security firms.

Moscow denies the hacking allegations. President Barack Obama last week ordered the expulsion of 35 Russian suspected spies and imposed sanctions on two Russian intelligence agencies he said were involved in hacking U.S. political groups such as the Democratic National Committee.

In a tweet on Wednesday, Trump was skeptical about a Russian role in the affair, writing: “(WikiLeaks founder) Julian Assange said ‘a 14 year old could have hacked Podesta’ - why was DNC so careless? Also said the Russians did not give him the info!”

However, on Thursday, Trump said in another post on Twitter that he was not against intelligence agencies or in agreement with Assange, whose organization leaked emails.

“The media lies to make it look like I am against ‘intelligence’ when in fact I am a big fan!” Trump tweeted.

Clapper said Assange had put American lives in danger and deserved no credibility. McCain and other lawmakers also blasted Assange.

Senator Claire McCaskill, a Democrat, said there would be “howls” from Republicans if a Democrat had described intelligence officials as Trump had.

Documents stolen from the DNC and Clinton’s campaign chairman John Podesta were posted on the Internet before the election, embarrassing the campaign.

U.S. intelligence officials have said Russian cyber attacks were specifically aimed at helping Trump beat Clinton. Several Republicans have acknowledged the Russian hacking but have not linked it to an effort to help Trump win.

Trump and top advisers believe Democrats are trying to delegitimize his victory by accusing Russia of helping him.

Senator Tim Kaine, an armed services committee member who was Clinton’s vice presidential running mate, called for Congress to act. “This is very serious,” Kaine said. “It is my hope that this congress is willing to stand in a bipartisan way for the integrity of the electoral process.”

Senator Lindsey Graham, a foreign policy hawk and former 2016 Republican presidential candidate, said Obama’s recent actions against Moscow fell short.

“I think what Obama did was throw a pebble. I’m ready to throw a rock,” Graham said, adding later: “Putin is up to no good and he better be stopped.”