The Department of Justice has given California authorities 48 hours to review a list of alleged noncitizen voters or face legal action, according to a letter Reuters reviewed, marking an unprecedented attempt to find evidence of voter fraud ahead of next month’s elections.

The letter, authored by acting Deputy Attorney General R. Trent McCotter and sent to California Secretary of State Shirley Weber, says federal officials gave Los Angeles County administrators a “password-protected thumb drive” containing the names of individuals the government “believes are likely noncitizens” who were issued mail-in ballots.

The directive requires the state to withhold ballots from those named on the list until it can verify their citizenship status. Flagged voters who have already received ballots must have their submissions “segregated” once returned, and ballots already submitted should not be counted “until citizenship has been determined,” the letter dictates.

Federal authorities “will have no choice but to seek emergency relief,” the letter warns, if state officials fail to follow these instructions.

“Allowing noncitizens to vote cancels citizen votes,” McCotter wrote in the communication.

The ultimatum reflects a heightened effort by the Trump administration to pressure states over alleged voting by non-citizens, signaling that the Justice Department may be prepared to challenge ballots in the weeks before the November 3 election, when Republicans will be defending narrow congressional majorities.

Justice Department spokeswoman Kiersten Pels confirmed the letter’s existence in a statement to Reuters, stating: “The department will take all necessary legal action to protect election integrity and ensure compliance with federal law.”

Spokespersons for the White House and the California secretary of state’s office did not immediately respond to requests for comment.

Under President Donald Trump, the Justice Department has repeatedly asserted that voting by noncitizens is a widespread problem, despite numerous studies demonstrating that the practice is rare.

A Reuters review in September found that while software glitches and clerical errors may have added more than 30,000 noncitizens to state voter rolls since 2000, it remained unclear how many had actually cast ballots.

Experts emphasized that these figures were far too small relative to the nation’s 170 million registered voters to influence election outcomes.

In July, the Justice Department dispatched letters to chief election officials across all 50 states, warning of potential criminal prosecution if they knowingly permitted noncitizens to vote or remain on voter rolls.

The department’s Civil Rights Division has requested nonpublic voter information, including birth dates and Social Security numbers, from nearly every state as part of its effort to identify noncitizens. It has filed lawsuits against 30 states led by both Democrats and Republicans that refused to release their rolls, losing 26 of those cases.

Reuters reported last month that the Department of Homeland Security was utilizing a commercial data broker to access sensitive voter details not included in public records, a practice that officials and experts warned could result in eligible citizens being mistakenly identified as illegal voters and potentially detained.

Federal prosecutors have also stepped up prosecutions against noncitizens accused of registering fraudulently or voting in previous elections, announcing 16 new criminal cases on Monday.

Officials in Democratic-led California have clashed for months with the Trump administration over federal efforts to intervene in state voting procedures based on allegations of voter fraud.

Homeland Security Secretary Markwayne Mullin informed California officials on July 16 that federal authorities believed noncitizens were present on the state’s voting rolls, according to the letter.

California could play a pivotal role in deciding control of the US House of Representatives, particularly after state voters approved a new congressional map favoring Democrats amid a nationwide redistricting push initiated in Republican-led states.

Voters in Los Angeles will also decide on a local ballot measure that would permit certain noncitizen legal residents to vote in municipal elections.