FBI Director James Comey told Congress there have been more "attempted intrusions" in U.S. voter registration databases, and the agency is looking "very, very hard" at Russian hackers who may be trying to disrupt the U.S. election.
The FBI in August warned the 50 U.S. states to boost their election security in light of evidence that hackers targeted online data systems in at least two states, Illinois and Arizona.
"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," Comey told the House Judiciary Committee on September 28. "There's no doubt that some bad actors have been poking around."
Comey did not say where the latest intrusions were found or provide further details. Unidentified U.S. law enforcement officials told CNN that they have discovered attempted hacks of voter registration sites in more than a dozen states and they believe Russia was behind them.
Comey was more explicit than ever in fingering Russia for trying to interfere in the U.S. electoral system. He said the FBI is "investigating to try to understand exactly what mischief the Russians might be up to in connection with our political institutions and the election system more broadly."
The FBI in a recent bulletin disclosed that 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.
Then in August, the FBI said hackers used the same vulnerability in an "attempted intrusion activities into another state's Board of Election system."
The FBI has stressed that no information in the system was changed by the hackers and none of the intrusions put state voting systems were at risk.
"We are urging the states just to make sure that their deadbolts are thrown and their locks are on and to get the best information they can from [the Department of Homeland Security], just to make sure their systems are secure," Comey said.
So far, some 18 states have sought help from the department to fend off potential cyberintrusions, officials said.
At a separate hearing on September 28, a Homeland Security official sought to assuage concerns among lawmakers about Russian interference in the election.
Homeland assistant secretary Andy Ozment said the U.S. government has confidence in the overall integrity of the nation's electoral system, which he said cannot easily be breached by cyberspies.
"Our voting infrastructure is diverse, subject to local control, and has many checks and balances built in," Ozment said.
He noted that states use a wide variety of voting machines and registration systems. They are not all connected to one another, and most aren't connected to the internet, he said.
Because of this, Ozment said he doubted a cyberattack could change the outcome of the national election on November 8.
Comey also stressed that because the U.S. voting system is highly decentralized and controlled by the states, a broad hacking of the U.S. election would be nearly impossible to carry out.
"The vote system in the United States is very, very hard for someone to hack into because it's so clunky and dispersed," he said.
With reporting by AP, CNN.com, and ABCNews.com