Voting by noncitizens is extremely rare, and election experts have raised concerns that the justice department’s crackdowns could disenfranchise US citizens.

The arrests came days after the justice department sued California, demanding the courts block the state from tabulating certain ballots in the high-stakes November midterms. The justice department had previously sent Los Angeles county election officials a thumb drive that purportedly contained the names of noncitizens who were unlawfully registered to vote. The justice department demanded that California officials, within 48 hours, confirm whether those on the list were citizens and set aside the ballots of those who remained unconfirmed.

The justice department’s lawsuit said it had sent California officials a list of more than 20,000 people who the federal government “believes are likely noncitizens” who were sent ballots by mail.

California’s secretary of state, Shirley Weber, has condemned the justice department’s demands as “outrageous”, saying federal officials had for months made “vague and unsubstantiated allegations” about alleged voter fraud in the state.