Russian hackers breached a state election network in Arizona, a state official said Monday night.
The FBI alerted Arizona election officials in June of the cyber attack, describing the threat as "credible" and ranked it as "an eight on a scale of one to 10," Matt Roberts, a spokesman for Arizona Secretary of State Michele Reagan, told the Washington Post on Monday. Reagan shut down the state’s voter registration network in response to the report.
The FBI investigators did not clarify whether the hackers were linked to the Kremlin or independent actors, Roberts said.
While the hackers did not compromise the state network, they stole the username and password of an election official in Gila County, located in central Arizona.
The FBI issued an alert to local election officials across the U.S. earlier this month advising them to ramp up cyber security measures after uncovering evidence that hackers breached two state election networks this summer, Yahoo News reported Monday morning.
Along with Arizona, hackers breached voter registration databases in Illinois. Federal officials said the Illinois attack was the first successful intrusion into a state voter registration database.
"This was a highly sophisticated attack most likely from a foreign entity," Kyle Thomas, director of voting and registration systems for the Illinois State Board of Elections, wrote to election officials in the state in July.
The FBI suspects the two intrusions in Arizona and Illinois were linked. The bureau is investigating whether hackers are attempting to launch widespread attacks against state election networks.
"The FBI is requesting that states contact their Board of Elections and determine if any similar activity to their logs, both inbound and outbound, has been detected," the FBI wrote in a flash alert.
The alert came as federal authorities are investigating a series of cyber attacks against non-governmental organizations, including the Democratic National Committee, thought to be the work of Russian government hackers.
Homeland Security Secretary Jeh Johnson held a conference call with election officials earlier this month to discuss how to make voting systems more secure, offering to provide federal security experts to examine vulnerabilities.
Johnson said the agency was not aware of "specific or credible cyber security threats" to the election at the time.
Cyber security officials fear the legitimacy of U.S. elections would be severely compromised if foreign governments are able to infiltrate voter data.