The FBI is investigating envelopes containing white powder that were sent to the offices of senators and congressmen in Alabama.
The envelopes set off anthrax scares in five Alabama cities on Monday.
Investigators shut down two federal courthouses and detained Rep. Jo Bonner, R-AL, in his office while they tested the powder.
"Each letter contained a small bag with a white powdery substance, and neither of these bags were opened," said Mike Lewis, a spokesman for Bonner.
So far, none of the letters have tested positive for anthrax or other hazardous substances.
The FBI and U.S. Postal Inspection Service said letters also were sent to the offices of U.S. Rep. Mike Rogers and U.S. Sens. Jeff Sessions in east Montgomery and Richard Shelby in the federal courthouse in downtown Montgomery. FBI spokeswoman Angela Tobon said all the letters sent to the lawmakers' offices appeared to be from a common source.
Chuck Spurlock, Sessions' state director, said employees notified the FBI of the suspicious letter and closed the office about noon. He said he received notification late Monday afternoon that the letter did not contain anthrax.
Shelby's spokesman, Jonathan Graffeo confirmed receipt of the letter, but said he had been asked by the FBI not to comment.
Rogers' press secretary Shea Snider said a letter was also sent to Rogers' Anniston office. Snider said the Anniston office was reopened after authorities determined the powder was not dangerous. She said the powder from the Montgomery office was still being tested late Monday. She said no one was injured in either of the offices.
Postal inspector Tony Robinson said the agency was offering a $100,000 reward for information leading to an arrest.