Richard L. Hasen, an election law professor at University of California at Irvine, said he could not recall ever seeing such an announcement.

“The Justice Department should not be a political tool, and this is a story that is going to be manipulated by the president to say his votes are being thrown out,” Hasen said.

Soon after the U.S. attorney’s statement was issued, the Trump campaign cited the case as evidence “Democrats are trying to steal the election.”

Earlier in the day, FBI Director Christopher A. Wray tried to reassure a Senate committee that although there are occasional instances of small-scale, local ballot fraud, the United States has not experienced an instance of widespread voter fraud by mail.