The Federal Bureau of Investigation has found breaches in Illinois and Arizona's voter registration databases and is urging states to increase computer security ahead of the Nov. 8 presidential election, according to a U.S. official familiar with the probe.
The official, speaking on condition of anonymity, said on Monday that investigators were also seeking evidence of whether other states may have been targeted.
The FBI warning in an Aug. 18 flash alert from the agency's Cyber Division did not identify the intruders or the two states targeted.
Reuters obtained a copy of the document after Yahoo News first reported the story Monday.
Accessing information in a voter database, much of which is publicly accessible, does not necessarily suggest an effort to manipulate the votes themselves. When registering, voters typically provide their names, home addresses, driver's license or identification numbers, and party affiliations.
But U.S. intelligence officials have become increasingly worried that hackers sponsored by Russia or other countries may attempt to disrupt the presidential election.
Officials and cyber security 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. Kremlin officials have denied that.
An FBI spokeswoman would not comment on the alerts but said the agency "routinely advises" on "various cyber threat indicators observed during the course of our investigations."