Federal authorities are investigating a series of suspicious letters mailed to election offices across the country, with some containing an unknown substance.

The FBI and U.S. Postal Inspection Service are working together to investigate the mailings, which were sent to various addresses nationwide, including secretary of state offices and other election-related offices.

Georgia is not among the states that received the suspicious letters, according to the Georgia Secretary of State’s Office.

In a statement, the FBI confirmed that investigators are working with law enforcement agencies to respond to the incidents and identify those responsible.

“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.

Federal officials have not confirmed which states received the letters.

Investigators are continuing their efforts to identify the individual or individuals responsible for sending the suspicious mailings.

The FBI and U.S. Postal Inspection Service say they remain committed to protecting the public and securing elections.