'Suspicious' mail sent around country with return addresses linked to the DSA triggers investigation

Senior Capitol Hill staff were warned in an email on Thursday that "letters containing suspicious substances" were mailed out to multiple congressional offices, as well as Secretary of State and their related election offices across the country. 

An image in the email showed one of the envelopes, which included a return address for the Democratic Socialists of America (DSA) headquarters. Though it's not clear the DSA had any involvement in the letters. 

A spokesperson for the DSA said in a statement to the New York Post that the organization hasn't been at the address postmarked on the envelope since 2013. 

The notice was sent from the U.S. Senate Sergeant of Arms to chiefs of staff, administrative managers and state directors. It stated that the U.S. Postal Service, the FBI and the U.S. Capitol Police were investigating the matter, Fox News reported. 

"Law enforcement officials are working to intercept any additional letters before they are delivered, however, it is possible that some letters may reach their destinations," the notice stated. 

"Some of the letters contained an unknown substance, and we are working closely with our law enforcement partners to respond to each incident," the FBI said in a statement to Fox.