The FBI issued a recent warning to state election officials to beef up their cybersecurity measures after foreign hackers broke into one state’s election system and targeted another.

The FBI confirmed to Fox News on Monday it issued a “flash” alert on Aug. 18, urging states to contact their Board of Elections if they suspect a breach.

In the flash alert from its cyber division, the FBI said it was investigating a pair of incidents and giving states specific signs to look out for to determine if their system had been hacked.

The FBI did not say which state was hacked or which one was targeted but says eight computers were involved.

Yahoo News, which first reported the story, identified the targeted states as Arizona and Illinois.

The FBI's Aug. 18 warning came after Homeland Security Secretary Jeh Johnson held a conference call with various state election officials to discuss cybersecurity issues.

"[T]he FBI routinely advises private industry of various cyber threat indicators observed during the course of our investigations,” the FBI told Fox News in a written statement. “This data is provided in order to help cyber security professionals and system administrators to guard against the persistent malicious actions of cyber criminals."

In the Illinois case, Yahoo reported officials were forced to shut down the state’s voter registration system for 10 days late last month after hackers downloaded the personal data of up to 200,000 of the state’s voters.

In the Arizona attack, Yahoo reported that the incident involved malicious software that was introduced into its voter registration system though the attack itself was not successful.

Fox News' Matthew Dean contributed to this report.