FBI director James Comey told a Senate panel that there was "penetration on the Republican side of the aisle and old Republican National Committee domains" no longer in use. Republicans have previously denied their organizations were hacked.
The testimony came in a Senate intelligence committee hearing that examined Russia's intrusions in the 2016 election campaign and its intentions, with America's top intelligence officials testifying just days after they released an unclassified report blaming Moscow for the hacks.
Comey later added that "there was evidence of hacking directed at state-level organizations, state-level campaigns, and the RNC, but old domains of the RNC, meaning old emails they weren't using. None of that was released."
Comey said there was no sign "that the Trump campaign or the current RNC was successfully hacked."
Asked by Sen. Mark Warner, a Virginia Democrat, whether the hacker had the ability to selectively leak that old information, Comey indicated that they did.
Comey also said that the Russians "got far deeper and wider into the (Democratic National Committee) than the RNC," adding that "similar techniques were used in both cases."
Comey said the FBI would have preferred to "get access to the original device or server" that was the target of hacking at the DNC. CNN previously reported that the Democratic National Committee "rebuffed" a request from the FBI to examine its computer services after it was allegedly hacked by Russia during the 2016 election.
The FBI instead relied on the assessment of a third-party security company called CrowdStrIke. Comey told senators that the "highly respected private company eventually got access and shared" the evidence with the FBI.
Director of National Intelligence James Clapper told the Senate committee, examining the cyber breaches, that the intelligence community concluded with "high confidence" that Russia hacked the election to "denigrate" Democratic presidential candidate Hillary Clinton and contrast her unfavorably to Republican Donald Trump.
"We have multiple high-quality sources that contribute to that assessment," Clapper said. "Attributing cyber operations is difficult but not impossible."
When Arkansas Republican Sen. Tom Cotton asked whether Russian President Vladimir Putin thought Trump was going to be the likely winner, Clapper responded, "Initially, no. They thought he was a fringe candidate and didn't think that at all."
Clapper said the intelligence community's report on the hacking was based on a variety of sources, including technical data, open-source information and human sources as well. He declined to offer more detail on how the information was collection in order to protect sources.
CIA Director John Brennan said that had called the director of a Russian intelligence agency behind the hacks and warned the Russians that they were playing with fire, a message the Russian said he'd pass on to Putin.
Clapper said that the response to activist attacks of this sort has to be carefully considered.
"It's not our call what to do in response," he said of the intelligence community. He said that policy makers and politicians should "consider the whole range of tools to respond."
"The challenge you get into with cyber for cyber, of course, you also have to consider the counter-retaliation to that," Clapper said. "While we spend a lot of time worrying about precision and being surgical, our adversaries" might not be that careful.
Clapper was asked whether intelligence agencies had found that people close to Trump had business interests that would make the Russians predisposed to favor Trump.
"The Russians just believed, or came to the conclusion, that because the President-elect is a businessman, that he would be easier to make deals with than the Democrats," Clapper said.
There were frequent questions about ties between the Trump campaign and Russia during the hearing. Earlier on, Comey refused to comment on whether the FBI is investigating any potential connections.
"I would never comment on investigations," Comey told Sen. Ron Wyden, an Oregon Democrat who repeatedly pushed the FBI director to release any information it had before Inauguration Day.
"If it doesn't happen before Jan. 20, I'm not sure it's going to happen," Wyden said.
But Sen. Angus King of Maine, an Independent, alluded tartly to Comey's very public statements about investigations into Clinton during the election campaign -- "the irony of you making that statement I cannot avoid."
Comey said that there was a difference between open and closed investigations. He pushed back on King, saying, "You asked me if you have any pending investigations and we're not going to talk about that."
He added that "I'm tone deaf on politics and that's as it should be."
Tuesday's hearing is the first public examination of Russia's activities since Trump received a briefing from the country's top intelligence officials on Moscow's interference in the presidential election.
The 17 US intelligence agencies already issued a statement in October expressing their unanimous assessment that Moscow had probed state election voting centers and penetrated the email accounts of Democratic groups and individuals. Much of that information was made public through the organization WikiLeaks, lead by Julian Assange.
In the classified and declassified reports released last week, they outlined their findings that Putin directly ordered the "influence campaign."
Republicans and Democrats both called for a thorough investigation into Russia's hacking of the presidential election Tuesday, reaffirming their confidence in US intelligence agencies' findings even as Trump has continued to downplay them.
The report "gives me no reason to doubt the findings," said the chairman of the Senate intelligence committee, North Carolina Republican Richard Burr. "We owe it to the American people to do an independent and bipartisan review."
After the conclusion of the public session, Burr said that the hearing would continue in a classified setting.
Congressional Democrats are pushing for a bipartisan, independent commission to look into the hacks. California Democratic Sen. Dianne Feinstein said last week that she and other senators will soon introduce a similar bill, while House lawmakers introduced one in early December and re-introduced it again last week.
Republicans have preferred to limit investigations to congressional committees they control. On Tuesday, the ranking Democrat on the House intelligence committee, California's Adam Schiff, said "the seriousness of these allegations warrant the most thorough and non-partisan of investigations."
Another California Democrat on the committee, Eric Swalwell, said the "best way forward is to have an independent, bipartisan commission that can depoliticize this. The Democrats were the victims this time," but "it could be the Republicans next time. The constant should be that both parties care about our democracy. There's plenty of room for Republicans to come on board."
In the final run-up to Nov. 9, Russia shifted its focus from influencing the campaign to undermining the validity of the results, a theme that Trump himself began to stress on the campaign trail, telling people the election was going to be "rigged."