Threatening Letters Sent to Congressional Offices, Colbert and Stewart

(FOX News) - Three congressional offices received threatening letters containing a powdery substance, officials said Wednesday, warning that more could be on the way.

While authorities have not yet confirmed which lawmakers were sent the letters, a Senate official told FOX News Channel that freshman Sen. Ron Johnson's (R-Wis.) state office was targeted.

Initial testing of the letters found that the powdery substance tested negative for containing a bio-agent, but a Capitol Hill official said subsequent letters may contain harmful materials.

"Although all letters received thus far have proved harmless, it is essential that we treat every piece of suspicious mail as if it may, in fact, be harmful," Senate Sergeant at Arms Terrance Gainer warned in an email sent to lawmakers.

FBI spokesman Jason Pack told FOX the agency was aware of this situation and responding accordingly.

"So far, none of the letters have contained a hazardous substance," Pack said. "We are working with those law enforcement agencies affected to determine if the mailings are related. We take these matters seriously and will investigate fully."

Two Senate state offices and one House of Representatives district office received the envelopes.

Meanwhile, a law enforcement official told FOX that similar threatening letters were sent to comedic news anchors Jon Stewart and Stephen Colbert at the Viacom headquarters in New York.

Earlier Wednesday, the Office of the Sergeant at Arms sent an outdated message in error warning that suspicious letters had been intercepted. It then retracted the warning, originally sent in 2011, and issued a new alert about potentially dangerous mail.