Memos were sent to congressional staffers Thursday notifying them that the FBI, U.S. Postal Inspection Service, and Capitol Police had launched a probe into letters containing a suspicious powdery substance that were sent to election and congressional offices throughout the country.

One of the letters arrived at the office of Sen. Chris Murphy (D-Conn.) in Hartford, his spokesperson confirmed.

Election offices in Louisiana and Oklahoma were evacuated after parcels containing the mysterious substance were delivered, but the U.S. Postal Service intercepted others heading to Ohio, Maryland, Mississippi, and South Dakota.

One of the letters bore the acronym “DSA” and a return address associated with the Democratic Socialists of America.

“The FBI and U.S. Postal Inspection Service are investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices,” an FBI spokesperson said.

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident. We are also working with our partners to identify the individual or individuals responsible for the letters,” the spokesperson added. “This FBI will continue to safeguard the public and secure elections for American citizens.”