Senate Minority Leader Harry Reid (D-NV) asked the FBI on Monday to probe evidence that suggests Russia may be using cyber attacks in an effort to influence the U.S. presidential election.

Reid told the New York Times that recent classified intelligence briefings left him with fears that Vladimir Putin’s “goal is tampering with this election.”

“I have recently become concerned that the threat of the Russian government tampering in our presidential election is more extensive than widely known and may include the intent to falsify official election results,” the Nevada senator wrote in a letter to FBI Director James Comey.

On Monday, Yahoo News reported the FBI had alerted states to possible intrusion by foreign hackers targeting the vast databases of state election boards. Although officials did not name the states affected in recent attacks, an anonymous source confirmed to Yahoo that foreign hackers breached systems in Illinois and Arizona.

The attack in Illinois prompted state officials to clamp down its voter registration system for 10 days in late July after hackers were able to download the personal data of up to 200,000 voters, Ken Menzel, general counsel for the state’s Board of Elections, told Yahoo News.

A spokesman for the Arizona secretary of state told the Times that the FBI had told state officials Russians were to blame for the intrusion.