Federal agents with the Department of Homeland Security have been knocking on doors in several states to ask residents about their immigration status and if they've registered to vote or voted in past elections. 

The operation is part of a broader election-integrity effort to prevent noncitizens from voting ahead of the midterms, CBS News reported. 

The Northern California office of Homeland Security Investigations posted photos on X in September showing agents knocking on doors with the comment that "HSI in Northern California will investigate any allegations of noncitizen voting and is committed to ensuring the enforcement of federal election law." 

The Kansas City office of HSI also posted photos of its agents knocking on doors with the comment, "AMERICAN ELECTIONS ARE FOR AMERICANS." 

The number of home visits increased in September and continued over the past weekend, according to CBS. 

The Department of Justice on Monday announced a string of arrests related to alleged voter fraud in California.

"This causes real damage to real elections and real damage to American citizens," a DOJ official said in a press conference. The DOJ also announced that it was bringing charges against 28 noncitizens illegally registered to vote, including some who did cast votes.

"These are not isolated instances," another official said, before confirming that the DOJ was reviewing thousands of alleged illegal voting cases across the country.

According to DHS, as many as 250,000 people illegally registered to vote, a figure some election experts have questioned. So far, the Justice Department has charged 156 people with voting-fraud offenses since the beginning of the second Trump administration.