A memo warned staffers to be cautious, but added that the substance is still under investigation and has not been determined to be dangerous.

“Unfortunately, there continues [sic] to be people out there who wish to express their views in this dangerous and alarming fashion,” the sergeant at arms said, “As long as we collectively adhere to the procedures, we have the best opportunity to remain safe from those who wish to do us harm.”

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 WickerRoger Frederick WickerMiss. governor names state's first female senator to replace retiring Cochran Co-founder of WhatsApp: 'It is time. #deletefacebook' Lawmakers zero in on Zuckerberg MORE (R-Miss.) and President Obama which tested positive for the deadly poison ricin. Dutschke has said he is innocent of the charges.