Two Democratic senators on Wednesday asked major vendors of U.S. voting equipment whether they have allowed Russian entities to scrutinize their software, saying the practice could allow Moscow to hack into American elections infrastructure.

The letter from Senators Amy Klobuchar and Jeanne Shaheen followed a series of Reuters reports saying that several major global technology providers have allowed Russian authorities to hunt for vulnerabilities in software deeply embedded across the U.S. government.

The senators requested that the three largest election equipment vendors - Election Systems & Software, Dominion Voting Systems and Hart Intercivic - answer whether they have shared source code, or inner workings, or other sensitive data about their technology with any Russian entity.

They also asked whether any software on those companies’ products had been shared with Russia and for the vendors to explain what steps they have taken to improve the security of those products against cyber threats to the election.

The vendors could not immediately be reached for comment. It was not immediately clear whether any of the vendors had made sales in Russia, where votes are submitted via written ballots and usually counted by hand.

“According to voting machine testing and certification from the Election Assistance Commission, most voting machines contain software from firms which were alleged to have shared their source code with Russian entities,” the senators wrote. “We are deeply concerned that such reviews may have presented an opportunity for Russian intelligence agents looking to attack or hack the United States’ elections infrastructure.”

U.S. voters in November will go to the polls in midterm elections, which American intelligence officials have warned could be targeted by Russia or others seeking to disrupt the process.

There is intense scrutiny of the security of U.S. election systems after a 2016 presidential race in which Russia interfered, according to American intelligence agencies, to try to help Donald Trump win with presidency. Trump in the past has been publicly skeptical about Russian election meddling, and Russia has denied the allegations.

Twenty-one states experienced probing of their systems by Russian hackers during the 2016 election, according to U.S. officials.

Though a small number of networks were compromised, voting machines were not directly affected and there remains no evidence any vote was altered, according to U.S. officials and security experts.