"There have been a variety of scanning activities, which is a preamble for potential intrusion activities, as well as some attempted intrusions at voter registration databases beyond those we knew about in July and August," he told the House Judiciary Committee. "There's no doubt that some bad actors have been poking around."
In August, the FBI issued a warning to state governments noting that hackers infiltrated the Illinois State Board of Elections two months earlier and then tried to breach election systems in Arizona.
Sources have told ABC News that Russian hackers were likely behind the cyberattacks.
Speaking to lawmakers today, Comey said the FBI is trying to figure out "just what mischief is Russia up to in connection with our election."
He emphasized that voter registration databases — not the voting system itself — are being targeted by hackers.
He added, "That is work that goes on all day, every day."
During a separate House hearing, a top Department of Homeland Security official, the head of the U.S. Election Assistance Commission, and the secretary of state of Georgia all said a cyberattack could not change the outcome of the 2016 election. A cyber-expert from New York University agreed, calling it unlikely, but a Princeton University professor described it as possible.
Dr. Andy Ozment, the assistant secretary for cybersecurity and communications at DHS, said hackers who broke into the voter registration system in Illinois and targeted a similar system in Arizona appear to have been looking to copy the personal information in those databases and perhaps then sell that information online. The aim was apparently not to affect the election process, he said.
"We have not seen intrusions intended to in any way impact individuals' votes and actual voting," Ozment said.
Many suspect Russian hackers are also to blame for these cyberassaults on Democratic organizations.
In late June an "unknown actor scanned a state's Board of Election website for vulnerabilities" and, after identifying a security gap, exploited the vulnerability to conduct a "data exfiltration," or unauthorized data transfer, the FBI said in a recent bulletin.
Then in August, hackers used the same vulnerability in an "attempted intrusion activities into another state's Board of Election system," the FBI said.