Russian Hackers Targeted Nearly Half of States' Voter Registration Systems, Successfully Infiltrated 4 The cyber-attacks are greater than U.S. officials have acknowledged so far.

 -- Nearly half of the states in the U.S. have recently had their voter registration systems targeted by foreign hackers, and four of those systems have successfully been breached, sources tell ABC News.

That amount of targeting and actual infiltration into state election-related systems is significantly larger than the U.S. government has been willing to acknowledge.

Hackers working on behalf of the Russian government are suspected in the onslaught against more than 20 state election systems, according to sources with knowledge of the matter.

"There's no doubt that some bad actors have been poking around," FBI Director James Comey told lawmakers Wednesday, without offering any more specifics.

He acknowledged there have been “some attempted intrusions at voter registration databases” since August, when the FBI issued a bulletin to state governments warning that hackers had infiltrated the Illinois State Board of Elections and tried to breach election systems in Arizona.

Testifying before the House Judiciary Committee, 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.

"This is very different than the vote system in the United States, which is very, very hard for someone to hack into because it's so clunky and dispersed," Comey said, adding that states should be in contact with the Department of Homeland Security and "make sure that their deadbolts are thrown and their locks are on."

During a separate House hearing on Tuesday, Homeland Security Secretary Jeh Johnson said 18 states had reached out to his department seeking assistance in protecting their election systems.

Meanwhile, another top Homeland Security official and the head of the U.S. Election Assistance Commission both said a cyberattack could not change the outcome of the 2016 election.

Dr. Andy Ozment, the assistant secretary for cybersecurity and communications at DHS, told lawmakers on Wednesday that the 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.

For months, the FBI has been investigating what appear to be coordinated cyberattacks on Democratic organizations -- the most damaging so far being the hack of the Democratic National Committee.

Not only did the hack apparently allow cyberoperatives to steal opposition research on Republican nominee Donald Trump, but many suspect it also led to the theft of internal messages that appeared to show efforts by DNC officials to undermine Democratic presidential candidate Bernie Sanders during the primary season.

After those damaging emails were publicly released by WikiLeaks, Florida Rep. Debbie Wasserman Schultz stepped down as the DNC's chairwoman. 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.

"The prospect of a hostile government actively seeking to undermine our free and fair elections represents one of the gravest threats to our democracy since the Cold War,” Senate Minority Leader Harry Reid, , D-Nev., wrote in a recent letter to Comey.

Asked this summer why Russia might be trying to undermine the U.S. political process, Director of National Intelligence James Clapper said Russian President Vladimir Putin is "paranoid" about the potential for revolutions in Russia, "and of course they see a U.S. conspiracy behind every bush, and ascribe far more impact than we’re actually guilty of."

"They believe we’re trying to influence political developments in Russia, we’re trying to affect change, and so their natural response is to retaliate and do unto us as they think we've done to them," he said.