Hackers based in Russia were behind two recent attempts to breach state voter registration databases, fueling concerns the Russian government may be trying to interfere in the U.S. presidential election, U.S. intelligence officials tell NBC News.
The breaches included the theft of data from as many as 200,000 voter records in Illinois, officials say.
The incidents led the FBI to send a "flash alert" earlier this month to election officials nationwide, asking them to be on the lookout for any similar cyber intrusions.
One official tells NBC News that the attacks have been attributed to Russian intelligence agencies.
"This is the closest we've come to tying a recent hack to the Russian government," the official said.
That person added that "there is serious concern" that the Kremlin may be seeking to sow uncertainty in the U.S. presidential election process.
Two other officials said that U.S. intelligence agencies have not yet concluded that the Russian government is trying to do that, but they are worried about it.
They said the Russians have long conducted cyber espionage on political targets. The question now is whether they are moving into a covert intelligence operation designed to destabilize the U.S. political process.
The alert, first reported by Yahoo News, provided IP addresses associated with the hack attempts, though it did not mention Russia.
One of the IP addresses was involved in both breaches, the FBI alert said.
"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 alert said.
The bulletin does not identify the targeted states, but officials told NBC News they were Illinois and Arizona. Illinois officials said in July that they shut down their state's voter registration after a hack. State officials said Monday the hackers downloaded information on as many 200,000 people.
State officials told the Chicago Tribune they were confident no voter record had been deleted or altered.
In Arizona, officials said, hackers tried to get in using malicious software but were unsuccessful. The state took its online voter registration down for nine days, beginning in late June, after malware was discovered on a county election official's computer. But the state concluded that the system was not successfully breached.
Those incidents led Homeland Security Secretary Jeh Johnson to host a call earlier this month with state election officials to talk about cybersecurity and election infrastructure.
Johnson said DHS isn't aware of any specific cyber threat against election-related networks, but he urged officials to examine how to better secure their systems, according to a summary of the call put out by the department.
U.S. intelligence officials have previously said Russian intelligence agencies were behind hacks into the Democratic National Committee and related organizations. There has been a long running debate among intelligence analysts about what Russia is up to.
Voting systems have not been considered "critical infrastructure," by the Department of Homeland Security, so they are not subject to federal government protections.
Independent assessments have found that many state and local voting system are extremely vulnerable to hacking.