The Justice Department is pressing California election officials to set aside mail-in ballots from people it believes are noncitizens. It is warning that officials who count those ballots could face criminal charges.

Federal prosecutors sent letters Friday to county election offices across the state, giving them 48 hours to work with federal agencies to determine who on a government list is a citizen. NBC News reported that the letters went to 57 of California's 58 counties. Los Angeles County received a similar letter last week.

The secretary of state and Los Angeles County have refused the demand, and other counties are weighing their response. The fight is now before a federal appeals court, 10 days before Election Day.

Here's what to know.

The department wants counties to "segregate — not cancel" the mail-in ballots of people on its list until their citizenship can be confirmed, according to a letter to San Bernardino County obtained by NBC News. That county's list included 1,133 names.

It also asked counties to send identifying information about the voters on the list, including Social Security numbers and copies of their registrations, the AP reported.

"Any election official who knowingly processes the ballot of a noncitizen is opening themselves to legal jeopardy," said Bill Essayli, the first assistant U.S. attorney in Los Angeles, who signed the letters.

The Department of Homeland Security compared California's voter registration files with federal immigration records. It flagged more than 20,000 people it says are "likely" noncitizens who are registered to vote, according to a Justice Department release. Under California law, every registered voter is mailed a ballot.

Only U.S. citizens may vote in federal and state elections. "Even one noncitizen vote cancels out the ballot of an American citizen," Homeland Security Secretary Markwayne Mullin said in the release.

Essayli says the request is limited. "We're not asking California to take any drastic steps other than to sequester the ballots from the suspected noncitizens," he told Fox News.

State officials say the federal data hasn't been verified, and that acting on it could keep eligible citizens from voting. In court filings, lawyers for the state argued that counties lack both the authority and the federal records needed to check citizenship. They also said the demand could delay certification of the results.

Some ballots can't be pulled back at all. Once a mail-in ballot's signature is verified, it is separated from its envelope and counted with the others. Hundreds of thousands of ballots have already been processed, according to court filings.

Dean Logan, the county's top election official, said the letters carry "a posture of intimidation and threats" just before the election, CNN reported. Sen. Alex Padilla, a Democrat of California, said the administration "has not provided any evidence for their claims of massive voter fraud."

Cases do happen, but reviews have found very few. In 2024, Georgia Secretary of State Brad Raffensperger, a Republican, audited the state's voter rolls and found 20 noncitizens among about 8.2 million registered voters.

Federal claims have not always held up. In August, DHS told Nevada officials it had flagged about 15,900 possible noncitizen registrants. It later said that number was preliminary and that it had confirmed only 185 through manual review, CNN reported. In September, Nevada officials said their own review found that all 185 were U.S. citizens. DHS disputes that finding.

For now, nothing has changed. No court has ordered California to set aside any ballots, and counties are still processing the ballots voters return. Voters can check the status of their ballot through the secretary of state's ballot tracking tool.

The Justice Department sued California and Los Angeles County on Oct. 15 to block the ballots from being counted. U.S. District Judge David Carter scheduled a hearing for Tuesday. The department asked the 9th U.S. Circuit Court of Appeals to step in sooner, and the appeals court ordered California to respond by noon Saturday.