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.