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NEW: The FBI and U.S. Postal Inspection Service are looking into a series of suspicious mailings sent to state election officials and congressional offices. Officials say some of the mailings contained a powdery white substance.

It is not clear who sent the mail, which was sent to election officials in states including Louisiana, Ohio and Oklahoma as well as congressional offices.

According to a statement from Louisiana Secretary of State spokesperson Trey Williams, an employee who opened a letter sent to the office on Tuesday was not injured but was taken to the emergency room as a precaution. Law enforcement found no evidence of harmful substances.

According to a communication from the Senate's Sergeant at Arms (SAA) to congressional staff I obtained, 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."

"Some of the letters have contained an unknown substance, and we're working closely with our law enforcement partners to respond to each incident," Donahue said.

The SAA's note to staff said that all suspicious letters, packages, or substances "should be treated as potentially hazardous and handled in accordance with established safety protocols," and warned staff not to handle suspicious letters and leave the area where they are discovered.

The note said the mail could be indicated by discoloration, powder emanating from the envelope and postmarks that do not match the return address. An image included in the communication had a return address labeled "DSA" and a New York City return address.