The Trump administration on Monday asked Washington state to set aside any ballots received from more than 400 people it suspects registered to vote without U.S. citizenship.

In a letter to the Washington Secretary of State’s Office, the Justice Department said it had identified 1,500 votes cast by the over 400 people in prior elections. It asked the state to set aside any ballots the people might cast in next month’s midterm until state officials can confirm they are legally eligible to vote. The state has more than 5 million registered voters.

“We are asking the state to set aside those ballots until we can confirm they are legal voters,” Acting U.S. Attorney Neil Floyd said in a news conference.

Much of the list was built based on “recent contacts” with suspected noncitizens, Floyd said, because federal officers are now routinely checking to see if the people they interact with are registered to vote. His office wants to compare that list against the state’s voter information because that will help determine how many people actually cast ballots, Floyd said.

Without access to those state records, proving someone illegally voted will be a difficult and labor-intensive process, Floyd said.

Washington Secretary of State Steve Hobbs was reviewing the administration’s demand. His office noted that it had received about 60 subpoenas from the Justice Department since last month seeking information about voters.

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