Federal authorities have charged an Oklahoma man with sending hundreds of letters containing nonhazardous white powder to public officials before the 2024 and 2026 elections.

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

Robin Martin is charged with conveying a hoax and mailing threatening communications. Martin was expected to make his initial court appearance on 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 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 US Postal Inspection Service were investigating. The FBI referred questions to the US attorney’s office Tuesday, and the Postal Inspection Service didn’t immediately return an email message from the AP seeking comment.