Federal prosecutors on Friday ordered election officials across California to set aside and verify mail ballots from suspected noncitizens within 48 hours, the latest step in an escalating fight over the state’s voter rolls, according to the Los Angeles County registrar and a letter obtained by The Associated Press, as CBS News reported.

At least 18 counties received the letters, the Los Angeles County registrar’s office said. Kiersten Pels, a Justice Department spokeswoman, told MS NOW that the department had contacted every county in the state. The letters demanded Social Security numbers, copies of state IDs and voter registration applications for people on a federal list, and asked officials to sequester any ballot whose citizenship could not be verified. In San Bernardino County, the list named 1,133 people; Contra Costa County got 407 names. Both lists are drawn from a pool of more than 20,000 people the administration calls “likely” noncitizens.

“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 whose name appeared on the letters. In the San Bernardino letter, seen by CNN, Mr. Essayli wrote that officials who knowingly tabulate such ballots “risk becoming aiders and abettors to a federal crime.”

Kristin Connelly, the clerk-recorder of Contra Costa County, received a hand-delivered letter from U.S. Attorney Craig Missakian warning that she would face prosecution if she failed to “segregate” ballots from the people on the list. “It’s an unenforceable 48-hour deadline delivered late on a Friday,” she told MS NOW. “There’s nothing about this that is actionable for me at this moment, because I need to follow the state and federal laws.” Asked whether she feared prosecution, she said no: “Because I’m not doing anything wrong. I protect our voter rolls, we’re very meticulous about it.”

Ms. Pels said the department wanted each county to commit to working with the Department of Homeland Security to verify citizenship quickly. “With their help, we can ensure the most secure election in California history,” she said.

The letters followed an Oct. 13 demand from Acting Deputy Attorney General Trent McCotter to Los Angeles County and Shirley Weber, the secretary of state, and a lawsuit filed last week seeking to block officials from counting more than 20,000 ballots tied to people on the federal list. A hearing in the case is set for Tuesday, but the administration on Friday asked the U.S. Court of Appeals for the Ninth Circuit to rule by Saturday instead. “Every minute of processing that occurs without segregating the ballots of suspected ineligible voters causes untraceable disenfranchisement and dilution of the votes of United States citizens,” its motion said.

Dean C. Logan, the Los Angeles County registrar, told fellow clerks in a message on Friday that the letter “misrepresents the facts of their own actions” and “threatens local election officials twelve days before the election.” A preliminary sampling of the names on the Los Angeles list, Mr. Logan said, found that about 43 percent had canceled registrations and more than 70 percent had no voting history. In a court filing, the county said 25.8 percent of the sampled voters listed a place of birth in the United States, “a strong indicator of citizenship status.”

Lawyers for California have argued in filings that the demands threaten to delay certification of the vote and undermine the legitimacy of the November midterms, and that the federal data is unreliable enough to risk disenfranchising citizens. Voting is already underway: the state’s more than 23 million registered voters received mail ballots this month, and more than 330,000 had been counted. Illegal voting by noncitizens is rare, difficult to prove and has not been shown to change the outcome of any race.

Earlier in the week, federal authorities charged 28 people from the list of 20,000 with illegally registering to vote; some were also charged with illegally voting and lying on naturalization paperwork. The Justice Department’s Civil Rights Division has filed 31 lawsuits to force states to hand over unredacted voter rolls and acknowledged it intends to share the data with Homeland Security. It has lost 26 of those cases and has yet to win one, after Harmeet Dhillon, the division’s assistant attorney general, wrote to all 50 states in July threatening criminal action.

The federal lists have not held up well elsewhere. In Nevada, DHS told state officials that 15,900 registrants might be noncitizens, then said the figure was preliminary and that manual review had confirmed 185. Nevada’s review found all 185 were U.S. citizens, a conclusion the federal agency disputes. In August, Homeland Security Secretary Markwayne Mullin said his department had identified 250,000 suspected noncitizens on the rolls in California, Pennsylvania, New Jersey and Nevada. The administration is also fighting in Alaska to block certification of the Senate race if the Republican incumbent, Senator Dan Sullivan, loses.

Senator Alex Padilla, Democrat of California and the ranking member of the Senate Rules Committee, said the administration was “afraid and desperate.” “Trump’s DOJ has not provided any evidence for their claims of massive voter fraud,” Mr. Padilla said. “Election officials: We have your back. California: Your vote is your voice. Use it and let’s hold this administration accountable.”