The visits, which began ramping up in September, are part of a Department of Homeland Security effort to investigate suspected noncitizen voting. Internal documents describe field reviews in California, Georgia, Missouri, Pennsylvania, Washington and Wisconsin. Hundreds of Homeland Security Investigations agents were reassigned from other cases for the initiative, according to the documents.

The operation has raised concern among voting-rights and civil-rights advocates, who say armed agents at people’s doors could intimidate eligible voters, including naturalized citizens. Jessica Marsden, an attorney with Protect Democracy, said the federal government’s involvement in voter eligibility checks could be intended to frighten lawful voters away from participating. The group is suing DHS over the door-knocking campaign.

DHS has said there could be as many as 250,000 U.S. citizens illegally registered to vote, a figure election experts have disputed. The Justice Department has charged 156 people with voting-fraud-related offenses since the start of the Trump administration, according to the department. The United States has about 227 million registered voters.

The visits coincided with arrests in California. The Justice Department said Monday that federal agents had arrested more than 20 people, all green card holders, on allegations they illegally registered or voted. In one case, dozens of agents went to the home of a 72-year-old Belizean man, who was arrested in his pajamas and accused of voting illegally in four elections. The allegations have not been established in court.

DHS said its investigative arm, Homeland Security Investigations, is working to identify and prosecute election fraud. A department spokesperson said the agency is focused on ensuring that only U.S. citizens elect American leaders and that people in the country legally who are not breaking other laws have nothing to fear.

A former HSI agent, Eric Balliet, said he had never encountered agents being sent to private homes to check voters’ eligibility during his 23 years with the agency. HSI is part of Immigration and Customs Enforcement and typically investigates crimes including drug smuggling, money laundering and child exploitation.

The effort followed a June DHS directive ordering ICE to help deport alleged noncitizen voters. Agents have posted images and videos of home visits on social media, including footage of armed officers and a document labeled “Target List.”

An HSI agent familiar with the initiative told CBS News that investigators draw on government databases, voter rolls and other records, then compare names with citizenship information from U.S. Citizenship and Immigration Services. A whistleblower at USCIS said employees were asked to review information on tens of thousands of people, including citizens, and given a target of 40 cases a day. DHS denied that the work was rushed or subject to a quota for finding criminality.

Marsden said such database matching risks mistakenly identifying eligible voters. She pointed to an earlier DHS claim that nearly 16,000 noncitizens appeared on Nevada voter rolls; the agency ultimately did not provide evidence of a single illegal registration, she said.

The cases against people arrested in the current operation remain allegations. Advocates say the federal campaign’s effects could extend beyond those under investigation, as lawful voters weigh whether interacting with federal agents—or going to the polls—is safe.