The FBI says letters containing white powder that were sent to seven governors were all postmarked in Texas.
Tests showed the powder was not dangerous. But officials are still trying to determine what it was.
FBI spokesman Rich Kolko said Tuesday the Dallas FBI office is investigating who sent the letters to the governors' offices in Alabama, Michigan, Minnesota, Mississippi, Missouri, Montana and Rhode Island. He says governors' offices in all other states have been warned to be on the lookout for suspicious letters.
The FBI says that even though the letters are a hoax, sending them was a crime because law enforcement and public safety personnel were called away from other duties to respond.