A U.S. cybersecurity monitor on Monday described another breach of a voter election system just after after a leaked FBI report revealed two similar attacks.

In June, anonymous hackers stole administrative login credentials in an unnamed county that would have let them delete voter registration records and prevent citizens from casting ballots.

The information comes from the Multi-State Information Sharing and Analysis Center (MS-ISAC), which monitors cyber attacks against state and local governments and shares information with the FBI. MS-ISAC is supported by the Department of Homeland Security.

The attack in June targeted a county election official through a phishing email, according to Brian Calkin, vice president of operations for the Center of Internet Security, which runs MS-ISAC.

In an interview, Calkin said the phishing email contained a malicious keylogger that stole the official’s login credentials.

This gave the hackers administrative privileges to modify voter registration records in the county. If the records had been deleted, the affected citizens wouldn’t have been able to vote, Calkin said on Monday. Fortunately, the attack was detected and no records had been found altered.

On Monday, Arizona's state government said that its voting system had briefly been taken offline in June due to an attack similar to what MS-ISAC found.

"A credential used by a county user to access the Arizona Statewide Voter Registration System was compromised by malware," said Matt Roberts, spokesman for the Arizona secretary of state's office, in an email. "This credential was leaked by a hacker."

Also on Monday, a leaked FBI bulletin revealed that hackers had recently targeted two unnamed state election systems. The FBI said one of the hacking attempts involved a SQL injection vulnerability, a common attack point in websites, and allowed hackers to steal voter registration records.

The FBI bulletin cites data pulled from MS-ISAC and warns election officials to be on guard against hacking attempts.

Calkin said MS-ISAC has detected a total of three states targeted in recent voting-related cyber attacks, but he declined to say which.

However, he noted that the SQL injection hack described by the FBI probably wasn’t severe. Voter registration records are available to the public on request, so there’s little need to steal them, he said.

“The more concerning part is the potential to possibly modify someone’s voter records or delete them,” he said.

Calkin couldn’t say how often state election databases are targeted in cyber attacks. He also couldn't recall how hackers attempted to penetrate the election database in the third state.

"We see attacks literally every day, but it's not necessarily against voter registration information," he said.

The report of the intrusions comes as the FBI has been investigating hacking attempts into government groups. A high-profile data breach of the Democratic National Committee last month stoked fears that hackers may be trying to interfere with the U.S. presidential election. Some cybersecurity experts have blamed that attack on Russian state-sponsored hackers.

The FBI declined to comment on the breaches against the state election databases mentioned in its bulletin.

However, in addition to Arizona, Illinois is another state that has reported a recent hack of its election system. A data breach detected in July stole voter registration records for 200,000 people, said Ken Menzel, the general counsel of the state's board of elections.

Some of that stolen data includes driver license numbers and the last four digits of voters' Social Security numbers, he said. Most of the rest of the stolen data could be bought by a political committee for a small fee, Menzel said.