Congressional offices receive mailed threats WASHINGTON (AP) - Some congressional offices and media organizations have received threatening letters containing a suspicious powdery substance that was tested and proved to be harmless, the FBI and the Senate's top law enforcement officer said Wednesday. Sergeant-at-Arms Terrance Gainer said in a memo to Senate offices that the letters were sent to three state and home district offices. Letters were also sent to several media organizations. FBI spokesman Peter Donald said agents had responded to Viacom and at least one other location in New York. Preliminary tests showed that the powder did not pose a threat, he said. "So far, none of the letters have contained a hazardous substance," FBI Special Agent Jason 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. The letters tell the recipients that there is a "10 percent chance you have just been exposed to a lethal pathogen." Even though none of the letters that have been tested have contained harmful substances, Gainer told staff to be extra vigilant. "The author of these letters has indicated that additional letters containing a powdery substance will be arriving at more Senate offices and that some of these letters may contain an actual harmful material," Gainer's memo to Senate offices said. 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. The letters bore a return address from "The MIB" and were postmarked Portland, Ore. The threats raised memories of post-9/11 incidents that rattled Washington. In mid-November 2001, authorities closed two Senate office buildings after anthrax attacks on Congress. Those attacks came after four people - two postal workers in Washington, a New York City hospital worker and a Florida photo editor - died from exposure to anthrax. Also at that time, an unopened envelope sent to Sen. Patrick Leahy, D-Vt., resembled a letter mailed the previous month to then-Senate Majority Leader Tom Daschle, D-S.D. The Leahy letter was discovered in the 280 barrels of congressional mail quarantined after a Daschle employee opened a powder-filled envelope.