America's top spy chief has said he is "more resolute" than ever before that Russia directly interfered in the US presidential election.
Director of US National Intelligence James Clapper has said he is "very confident" that Moscow hacked Democratic Party institutions and operatives, and spread fake news during the campaign.
While giving testimony to the Senate Armed Services Committee, who are investigating the alleged interference, General Clapper stopped short of calling the meddling "an act of war."
He said a motive for the cyber attack would be made public next week.
- Watch ITV News Correspondent Martin Geissler's report from Washington:
In Thursday's testimony, General Clapper confirmed the findings of the October statement and that disinformation and fake news, along with hacking continues.
Senator John McCain of Arizona, the Republican chairman of the committee, said "every American should be alarmed" by Russia's meddling, which was an "unprecedented attack on our democracy."
General Clapper added that while the hacks "did not change the vote tallies or anything of that sort...we had no way of gauging the impact that, certainly the intelligence community can't gauge the impact, that it had on choices the electorate made."
He said: "There's no way for us to gauge that. Whether or not that constitutes an act of war, I think is a very heavy policy call that I don't believe the intelligence community should make, but it's certainly would carry in my view, great gravity."
In previous assessments, the intelligence community has said Moscow interfered in the election to help Republican Donald Trump win.
The hearing comes a day before the president-elect receives a briefing by the CIA and FBI directors - along with the head of national intelligence - on the investigation into Russia's alleged hacking efforts.
Mr Trump has criticised their findings, cast doubt on their assessments and even seemed to back WikiLeaks founder Julian Assange's claim that Russia did not provide him with hacked Democratic emails.
In tweets early on Thursday, Mr Trump backed away from his apparent embrace of Assange, blaming the "dishonest media" for portraying badly.
General Clapper and other US intelligence said President Barack Obama had received a report on the Russian interference and other foreign meddling in the US election.
They said Russia posed a "major" and "growing threat" to US government, military, diplomatic and commercial operations.
Lawmakers from both parties have spoken of their distrust of Mr Trump's praise of Russian President Vladimir Putin and efforts to heal the rift between the United States and Russia.
General Clapper said lawmakers would be briefed on the Russian hacking report next week and an unclassified version was tentatively scheduled to be released shortly after that.
President Obama struck back at Moscow in late December with penalties aimed at Russia's leading spy agencies which the US claims were involved.
When Mr Trump takes office in late January these are sanctions that could be rescinded.