OKLAHOMA CITY (AP) — Federal authorities have charged an Oklahoma man with sending hundreds of letters containing nonhazardous white powder to public officials ahead of the 2024 and 2026 elections.

The powders turned out to be yeast or flour, though not all letters have been fully tested, the FBI said in a court filing made public Tuesday.

Robin Martin is charged with conveying a hoax and mailing threatening communications. Martin was expected to make his initial court appearance Tuesday afternoon in Oklahoma City. Online records do not list an attorney to comment on his behalf.

Federal authorities announced last month that they were investigating letters with “suspicious substances” that had been mailed to congressional and election offices nationwide and urged recipients to treat the mailings as “potentially hazardous.”

A message from the Senate Sergeant at Arms obtained by The Associated Press said that law enforcement officials were working to intercept any additional letters, but “it is possible that some letters may reach their destinations.”

The FBI said at the time that the bureau and the Postal Inspection Service were investigating. The FBI referred questions to the U.S. attorney’s office Tuesday, and the Postal Inspection Service didn’t immediately return an email message from the AP seeking comment.

WBRZ previously reported that the Louisiana Secretary of State's Baton Rouge office received a mysterious letter with white powder on Oct. 13. According to the criminal complaint for Martin, his letters were sent the same week Louisiana received its envelope of white powder.