WASHINGTON--Several members of the U.S. Congress received mail threatening a biological attack and containing a suspicious powder later found to be harmless as law enforcement officials warned on Wednesday that more letters could be on their way.

  A number of media organizations and TV shows, including the New York Times and The Daily Show with Jon Stewart, received mail postmarked Oregon warning that letters had been sent to the Washington or local offices of all 100 U.S. senators and that 10 contained a deadly pathogen, a law enforcement source said.

  House of Representatives Speaker John Boehner, the top Republican in Congress, received a letter containing a powdery substance at one of his offices in his home state of Ohio, a Republican aide said, adding that the powder was harmless.

  Letters containing powder were also received at the home-state office of two senators, according to Senate Sergeant at Arms Terrance Gainer, the chamber's chief law enforcement officer. Tests found the powder was harmless, Gainer said in a statement. He did not identify the senators.

  In a notice to Senate staffers titled "Urgent: Suspicious Mail Alert," Gainer warned that the sender of the letters had "indicated that additional letters containing a powdery substance will be arriving at more Senate offices and that some of these letters may contain actual harmful material."

  He warned that special attention should be paid to letters sent from Portland, Oregon. "The FBI is aware of this situation and is responding accordingly. So far, none of the letters have contained a hazardous substance," said FBI spokesman Chris Allen in Washington. Allen said the FBI was working with other law enforcement agencies to determine if the mailings were related.

  The New York Police Department was made aware of the letters received by some media outlets on Tuesday night and alerted the FBI and the U.S. Capitol Police, a law enforcement source said. The letters to the news organizations did not contain any suspicious substances. They demanded an end to corporate money and lobbying in U.S. politics, an end to corporate personhood, and called for a new constitutional convention, the source said. The letters were signed "the MIB".

  Other media that received the letters were The Colbert Report, Washington Post, USA Today, NPR, the Chicago Tribune, the Los Angeles Times, the Seattle Post-Intelligencer, MacNeil/Lehrer Productions, This Week with Christiane Amanpour, Countdown with Keith Olbermann and Fox News.