Local election officials across California are confused and frustrated after receiving letters from US President Donald Trump's Justice Department demanding that county registrars sequester ballots that the federal government believes were sent to noncitizen voters ahead of the midterm elections.

The letters, which were sent Friday, ask for sensitive information, including Social Security numbers, on roughly 20,000 voters. It is illegal for noncitizens to to vote or cast a ballot. The administration has insisted that immigrants voting is a major problem nationwide, when in fact it is quite rare, as even its own investigations have shown.

“All of this is simply meant to raise huge doubts about the integrity of this election,” said John Tuteur, the registrar of voters for Napa County in California’s wine country.

Tuteur and other election officials who spoke to The Associated Press on Saturday said they have already started to count ballots that have trickled in from the state's 23 million registered voters.

“We’re not changing anything we’re doing. We’re proceeding with the election,” he said.

The federal government set a 48-hour deadline to provide information, and it warned that legal action could be taken against any official who knowingly counted a noncitizen's vote.

However, the California attorney general's office said the letters contained errors. For example, Mendocino County officials received a list of people who were not residents there, according to the state's court filing.

The letters are the latest escalation in a legal battle between Democratic-dominated California and Trump's Department of Justice ahead of the midterms. The federal government sued on Oct. 15, roughly three weeks before elections, asking a judge to bar the state from counting mail-in ballots from people the federal government has identified as likely noncitizens.

A panel of 9th Circuit appeals court judges was to hear arguments about whether California can be compelled to comply with the federal government’s demands.

Tuteur said that two uniformed Department of Homeland Security officers delivered a letter to his office late on Friday afternoon. The letter included the names of 42 people who the federal government accused of being noncitizens illegally registered to vote.

There are roughly 86,000 people registered to vote across the county of over 130,000 people, Tuteur said. Approximately 11,000 people had already cast a mail-in ballot as of Friday. In larger counties like Los Angeles, that number has already reached over 331,000 counted mail-in votes. He said his employees and his relatives were worried about what could come next. “My family’s upset, worried about what’s going to happen to me,” he said.

The letter sent to Clerk-Recorder Kim Grady's office in rural Amador County listed four people allegedly illegally registered to vote in the county. Grady was waiting on more guidance from the state about what she should do, and she wasn't sure if she would get any clarity before the deadline.

US Department of Justice spokesperson Kiersten Pels emphasized in a statement on Saturday that the department is explicitly “not requesting a purge, cancellation of registrations, or any programmatic action." Instead, Pels wrote, federal officials are asking for collaboration to “ensure the most secure election in California history.”The letters on Friday came on the heels of a statewide sweep by federal authorities on Monday, where 28 people from the list of 20,000 were charged with illegally registering to vote. Some were also charged with illegally voting and lying on naturalization paperwork. Those charges could lead to years in prison.

Bill Essayli, the top federal prosecutor in Los Angeles, said that the clerks who didn't comply with the government's demands could face prosecution as well.

“The letters speak for themselves. Any election official who knowingly processes the ballot of a noncitizen is opening themselves to legal jeopardy," Essayli said in a statement.

Noncitizen voting is very rare. For example, earlier this month Attorney General Todd Blanche announced federal charges against 10 people in Minnesota alleged to have voted illegally over the last three elections, a period in which more than 8 million ballots were cast in the state.

But Essayli said on Monday that even a small number of illegal voters could “dilute” the vote and undermine confidence in elections.

Some election officials who spoke to The Associated Press Saturday have questioned the accuracy of the federal government's list of 20,000 voters.

“I expect this accusation to fall apart like we saw recently in Nevada,” Kristin B. Connelly, the Contra Costa County clerk-recorder and registrar of voters said, referring to remarks Trump made in July, when he claimed the neighboring state had 1,500 noncitizen voters on its rolls. After his administration was asked to identify them, the number of actual registered noncitizens turned out to be zero.

Officials in Los Angeles County, who are named as defendants in the federal lawsuit, have echoed Connelly's concerns about the federal government's data. A preliminary analysis of 200 names from the statewide list of 20,000 found that over 40 percent had canceled registrations, according to a court filing from county officials on Wednesday. More than 70 percent of that sample had no voting history whatsoever, and nearly 4 in 10 were born in the US.