WASHINGTON — The FBI, US Postal Inspection Service and Capitol Police are investigating letters “containing suspicious substances” that were mailed to congressional offices, according to emails sent to Hill staff Thursday.

The warning included a photo of one of the suspicious envelopes that was postmarked “DSA” and gave a return address at the same Financial District building that the Democratic Socialists of America used to list on their membership forms.

“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 Senate email notice stated.

Some of the missives have also been intercepted at addresses other than congressional offices.

The FBI, Postal Inspection Service and Capitol Police did not immediately respond to requests for comment. The DSA dd not immediately respond to a request for comment.