Homeland Security Secretary Markwayne Mullin on Friday notified election officials in California, New Jersey, Nevada and Pennsylvania that the Department of Homeland Security has identified what it says are more than 250,000 potential non-citizens on the states’ voter rolls.

Mullin said the department’s preliminary review found as many as 190,832 potential non-citizens registered to vote in California, 35,152 in New Jersey, 15,903 in Nevada and 14,576 in Pennsylvania, according to the Department of Homeland Security.

“Election security is national security,” Mullin said in a statement. “Only Americans should be electing American leaders.”

The letters — sent to California Secretary of State Shirley Weber, New Jersey Secretary of State Dale Caldwell, Nevada Secretary of State Francisco Aguilar and Pennsylvania Secretary of State Al Schmidt — ask each state to respond within two weeks and indicate whether it will work with DHS to review the registrations and help ensure what the administration described as “free, fair, and honest elections.”

The announcement comes as President Trump gave a national address Thursday focused on election integrity. In the speech, he accused China of stealing the voter registration data of 220 million Americans.

DHS said recent actions it has taken on elections include requiring recipients of certain Federal Emergency Management Agency grants to comply with election integrity measures, as well as directing U.S. Immigration and Customs Enforcement to pursue stricter penalties — including deportation — for non-citizens who illegally vote.

Federal law prohibits non-citizens from voting in federal elections, and documented cases are rare.

Election officials and voting rights groups have long cautioned that voter list matches can produce false positives because of outdated records, name similarities or errors in underlying databases.

DHS did not release details describing how it identified the potential registrations.