The FBI is urging US election officials to increase computer security after it uncovered evidence that hackers have targeted two state election databases in recent weeks, according to a confidential advisory.
The warning was in an August 18 flash alert from the FBI's Cyber Division. Reuters obtained a copy of the document.
Yahoo News first reported the story on Monday, citing unnamed law-enforcement officials who said they believed foreign hackers caused the intrusions.
US intelligence officials have become increasingly worried that hackers sponsored by Russia or other countries may attempt to disrupt the November presidential election.
Officials and cybersecurity experts say recent breaches at the Democratic National Committee and elsewhere in the Democratic Party were likely carried out by people within the Russian government.
Following the 19,000 emails that were published by WikiLeaks, Democratic National Committee chairwoman Debbie Wasserman Schultz resigned. WikiLeaks has promised there would be more leaks about Hillary Clinton to come.
Kremlin officials have denied the allegations of Moscow's involvement.
Concerns about election computer security prompted Homeland Security Secretary Jeh Johnson to convene a conference call with state election officials earlier this month, when he offered the department's help in making their voting systems more secure.
The FBI warning did not identify the two states targeted by cyber intruders, but Yahoo News said sources familiar with the document said it referred to Arizona and Illinois, whose voter registration systems were penetrated.
Citing a state election board official, Yahoo News said the Illinois voter registration system was shut down for 10 days in late July after hackers downloaded personal data on up to 200,000 voters.
The Arizona attack was more limited and involved introducing malicious software into the voter registration system, Yahoo News quoted a state official as saying. No data was removed in that attack, the official said.
Governments around the world are looking to take elections into the digital realm.
In Australia, the state of New South Wales uses the iVote system, which is well on its way to replacing postal ballots, having handled 283,000 votes in the state's most recent poll.
"They were using it because postal voting isn't working anymore," NSW Electoral Commission CIO Ian Brightwell said last year.
"I think [iVote] has a place in the voting world simply because it's a complementary service to paper voting. I wouldn't want to see it becoming dominant."
The iVote system was found to be vulnerable to the FREAK flaw, concerns that were dismissed by Brightwell as overstated.
Western Australia is working towards using iVote at its March 2017 election, with the NSW Electoral Commission looking to get Victoria on board too.
Last week in a submission to the Victorian Electoral Matters Committee, Australia Post detailed its plans to use the blockchain as it moves into the business of running elections.
"In many ways voting is an ideal use case for blockchain technology application beyond crypto currency," said Australia Post State Director, Victorian government and Tasmania, Tim Adamson.
"A ballot would be cryptographically represented within the blockchain, with each vote linked to the voter through their preference choice stored within the blockchain in a way that anonymises and protects that information from being publically accessible."
In December, Brazil ended the deployment of electronic voting machines due to cuts in public spending.
For the first time since 2000, the nation will return to using paper-based ballots.