The FBI, U.S. Capitol Police and U.S. Postal Inspection Service are investigating “multiple letters containing suspicious substances that were mailed to various addresses nationwide, including multiple Congressional offices,” according to a notice sent to Senate staff on Thursday and described to ABC News. 

Investigators didn’t immediately disclose more details of the suspicious substances.

Federal law enforcement officers are working to intercept the letters, according to the notice.

The Senate Sargent at Arms warned in the notice that some letters could make it to congressional offices. 

The Senate Sargent at Arms recommended treating any suspicious letters with caution, and that suspicious letters contain multiple postage markings, according to the notice.

The Capitol Police referred ABC News to the FBI for comment. 

The FBI confirmed in a statement Thursday that it was working with the U.S. Postal Inspection Service in a probe over “a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices.”

Those mailings also “contained an unknown substance,” according to the FBI.

“We are also working with our partners to identify the individual or individuals responsible for the letters. The FBI and the U.S. Postal Inspection Service will continue to safeguard the public and secure elections for American citizens,” the FBI said in a statement. 

It is unclear whether these letters are connected to the letters recently sent to Secretaries of State’s offices around the country.