A Seattle federal building has become the latest to report a suspicious substance in their mail room, Wednesday.

The substance was found on the 29th floor of the Jackson Federal Building, the same building that houses Sen. Patty Murray's Seattle office. A hazmat crew is on scene and it is unknown if the substance is related to threats received by some congressional offices outside Washington D.C. and media organizations earlier on Wednesday.

Murray's office confirmed that the letter with the substance was sent to her office.

The Seattle Fire Department Hazardous Material Team determined the substance in the envelope was non-hazardous.

Other organizations received threatening letters with the substance that read there is a "10 percent chance you have just been exposed to a lethal pathogen."

Though the substance from the Federal Building in Seattle is still being tested, other substances tested 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. A district office of House Speaker John Boehner, R-Ohio, received one of the letters, spokesman Kevin Smith said.

A federal law enforcement official, speaking on condition of anonymity, said that so far fewer than 10 members of Congress had received letters.

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.

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 sender wants an "end to corporate money and 'lobbying,'" an end to corporate "personhood" and a new constitutional convention. The Associated Press obtained a copy of a letter.

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.

The Associated Press contributed to this report.