David Tuason, 46, of Pepper Pike faces up to three years in federal prison, the Cleveland Plain Dealer reported. He is to be sentenced in July.
The targets of his letters ranged from Supreme Court Justice Clarence Thomas to student athletes like the Strongsville High School football team and black athletes at St. Ignatius High School, where Tuason had been a student.
In a 2003 letter to Thomas, Tuason threatened to blow up the Supreme Court.
Although investigators said Tuason began sending threats 20 years ago, he was charged with letters sent between 2003 and 2008. He was arrested in March after threatening e-mails were tracked to his computer.
He threatened to kill or maim black men involved with white women.