Glenn E. Morgan Jr., 40, now faces a maximum punishment of 10 years in prison and a $250,000 fine, a department release said Monday.
"It is inconceivable that acts of hate like this one continue to occur in the year 2011," said Thomas E. Perez, assistant attorney general for the Civil Rights Division.
Morgan was accused of mailing a noose, photographs of black men being lynched and a photo of the murdered body of Nicole Brown Simpson to the unidentified couple.
The envelope he sent also contained threatening messages indicating that black men who marry white women should be lynched and white women who marry black men will share Nicole Brown Simpson's fate.
Nicole Brown Simpson was found murdered in June 1994. She was the former wife of professional football player O.J. Simpson, who was acquitted of her killing but found liable in civil court.