Los Angeles (AP) — Federal prosecutors sent letters to county registrars across California on Friday demanding that local officials set aside and verify ballots that the Department of Justice said may belong to noncitizens, according to the Los Angeles County registrar and a letter obtained by The Associated Press.

The letters are the latest escalation in a legal battle between California and President Donald Trump’s Department of Justice, which has recently focused on the state in its efforts to root out noncitizen voting ahead of the midterm elections. Data shows illegal voting by people who aren’t U.S. citizens is incredibly rare, difficult to prove and has not been shown to change the outcome of races.

Earlier this month, the federal government targeted Los Angeles County and the California Secretary of State’s office in a lawsuit that aimed to block them from counting the votes of roughly 20,000 people who are “likely” noncitizen voters. A hearing on the case is scheduled for Tuesday.

The Trump administration asked the 9th Circuit court of appeals to rule on the lawsuit barring a decision from the district judge assigned to the case. They set a deadline for Saturday.

Now, according to the LA County registrar’s office, at least 18 counties across the state have received similar letters. They have been given a 48 hour deadline to provide identifying information corresponding with ballots that the federal government believes were sent to noncitizens illegally registered to vote. The letter requests that county officials also sequester any ballots where citizenship cannot be verified.

“Any election official who knowingly processes the ballot of a noncitizen is opening themselves to legal jeopardy,” said First Assistant U.S. Attorney Bill Essayli.

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