Sheriff David Clarke had resigned as Milwaukee County sheriff on Thursday afternoon.
WTMJ-TV reported that Clarke resigned Thursday. Christenson says the letter doesn't give the reason for Clarke's resignation.
But Clarke has been fighting a series of scandals in his position as sheriff in Milwaukee County as well, including the death of a mentally ill man who was allegedly deprived of water in his cell.
Clarke, a tough-on-crime conservative Democrat on paper, had become a favorite among conservatives with his provocative social media posts and his blind devotion to Donald Trump during his 2016 campaign.
In May, during a radio interview, he said he had been offered a job as an assistant secretary in the Department of Homeland Security (DHS). He announced in May that he was under consideration for the job, but he pulled his name from consideration the next month.
Some Wisconsin conservatives had encouraged Clarke to challenge U.S. Sen. A black law enforcement official willing to condemn the primarily black protest movements springing up around the country was a valuable political weapon for deceased Fox News head Roger Ailes, Breitbart.com chairman and former Trump aide Steve Bannon, and other influential figures obsessed with white grievance politics. He recently published a memoir, "Cop Under Fire".
Clarke's spokeswoman did not immediately respond to telephone messages and emails seeking comment.
He was later accused of plagiarism in his master's thesis at the Naval Postgraduate School.
Seven workers at the county jail he oversees are at the center of a criminal investigation into the dehydration death previous year of an inmate who prosecutors say was deprived of water as punishment.