A memo warned staffers to be cautious, but added that the substance is still under investigation and has not been determined to be dangerous.
The memo added that while the letters differed, each suspicious envelope was postmarked from Omaha, Neb., and had a mailing date of July 25.
All mail to the Senate is tested by the body’s mail processing facility before being delivered to staff offices.
The suspicious letters are only the latest mail scare this year.
Authorities have charged a Mississippi man, James Everett Dutschke, of sending letters in April to Sen. Roger Wicker (R-Miss.) and President Obama which tested positive for the deadly poison ricin. Dutschke has said he is innocent of the charges.