The Justice Department has opened a criminal investigation in California as it continues its effort to prove that as many as 20,000 noncitizens are on the state’s voter rolls, a U.S. official confirmed to NBC News on Thursday.

The official said the Justice Department has issued federal grand jury subpoenas to local election officials across the state. It’s unclear how many subpoenas were sent out and exactly who received them.

The New York Times first reported the new subpoenas.

In response to an inquiry from NBC News on Thursday, the Justice Department declined to comment on “leaks about alleged grand jury subpoenas” but said federal courts have said voting unlawfully results in criminal exposure.

The U.S. official told NBC News that receiving a subpoena doesn’t mean a person is necessarily a target, just that the person may have information relevant to the investigation. The official added that the Justice Department rarely subpoenas people it intends to charge with crimes.

The new subpoenas follow a federal judge’s ruling Wednesday to deny the Trump administration’s request to segregate 20,000 mail ballots from alleged noncitizens in California, calling into question the accuracy of the administration’s process to identify such people.

Los Angeles County Registrar-Recorder and County Clerk Dean Logan touted the decision Thursday.

“At every step, the U.S. Department of Justice has taken a confrontational, adversarial approach. Election officials are acting in accordance with state and federal law,” Logan said in a statement, adding that “clear court rulings” by both the federal judge and an appeals court have confirmed that.

Last week, Justice Department officials sent letters to 57 of California’s 58 counties warning local election officials to cooperate with federal investigations into illegal voting or face investigations themselves.

The Justice Department expanded the efforts this week, sending similar letters to officials in Washington state and asking its secretary of state to review a list of 400 registrants whom it believes to be noncitizens. It also asked that the state sequester any ballots those people cast.

The Trump administration faced two legal setbacks in the efforts this week when an appeals court denied its request Monday to block California from counting mail ballots from the alleged noncitizens and a federal judge Wednesday rejected its request to segregate those ballots.

The Trump administration has pressured state officials to assist in investigations of allegations of voter fraud for months, having sued 30 states and the District of Columbia for refusing to hand over their voter registration list data.

While noncitizens can appear on voter rolls, it is rare. In addition, newly naturalized citizens are often mistakenly flagged as noncitizens, especially since there is no comprehensive list of U.S. citizens.

Election experts have argued that the Trump administration’s lawsuits rely on flimsy legal arguments, particularly as the Justice Department doesn’t have legal authority to intervene in election certification.