Michigan Senator Carl Levin says his regional office in Saginaw received a suspicious letter and that authorities have been alerted.
Levin says in a statement that an aide received the letter Wednesday, but did not open it. Authorities are now investigating.
The Democratic lawmaker says he and his staff do not know if the mail presented a threat.
Separately, the FBI says a substance in letters sent to President Barack Obama and Mississippi Senator Roger Wicker have preliminarily tested positive for ricin, a potentially fatal poison.
The FBI says those letters are related and are both postmarked out of Memphis, Tennessee, dated April 8. Both letters were intercepted at off-site mail facilities.