The FBI is currently investigating digital attacks on voter registration systems in both Arizona and Illinois, according to an FBI flash bulletin obtained by Yahoo News. The Illinois attack took place in late July, bringing down the state’s voter registration for 10 days and stealing data on as many as 200,000 registered voters. The Arizona attack was less significant, introducing malware into the voter registration system but not successfully stealing any data.

The Department of Homeland Security offered cybersecurity assistance to a number of states in the wake of the attack, although it’s unclear if any systems changed as a result.

It’s unlikely that either attack could be used to alter voting totals in an actual election, although they could potentially be used to introduce irregularities into the state’s voter registration rolls. Still, the provisional ballot system makes it unlikely those irregularities would prevent any votes from being cast.

Groups like the Institute for Criticial Infrastructure Technology have also expressed concern over the integrity of voting machines themselves, although so far no evidence has surfaced of direct attacks on the machines. On Sunday, the ICIT released a report on vulnerabilities in the various voting systems currently in use, issued under the alarming title "Hacking Voting Machines is Easy!"

The FBI bulletin does not attribute the two attacks to any particular group, although Yahoo News traces a few pieces of evidence that link the attack to Russia. Certain methods used in the hack overlap with previous attacks linked to Russia, and at least one IP address used in the hack had surfaced previously in cybercrime forums.

That’s mostly circumstantial evidence, not nearly enough for a firm attribution, but it plays into long-standing concerns about the possibility of foreign digital attacks against the US election process. A number of experts raised concerns about politically motivated hacking after the release of hacked emails from the DNC earlier this year, which some saw as a Russian effort to damage Hillary Clinton’s presidential campaign.