Michigan Sen. Carl Levin received suspicious letter

(CNN) -- Authorities are investigating a suspicious letter at the office of Sen. Carl Levin in Saginaw, Michigan.

"Earlier today, a staffer at my Saginaw regional office received a suspicious-looking letter," Levin said in a statement. "The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating. We do not know yet if the mail presented a threat."