Several U.S. members of Congress were targeted with a biological attack warning through mail containing a suspicious powder.

Several media organizations and television show hosts, including the New York Times and Jon Stewart of The Daily Show, also received letters which claimed that 10 mails, containing a deadly pathogen had been sent to various U.S. senators' offices. The suspicious powder later turned out to be harmless.

House of Representatives Speaker John Boehner 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, according to a Reuters report.

Senate Sergeant at Arms Terrance Gainer said that two more senators had received the powdery substance in a mail postmarked Portland, Oregon. However, he declined to identify the senators.

Gainer sent a warning to the Senate staffers titled Urgent: Suspicious Mail Alert, indicating 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.

A U.S. law enforcement official, who wished to remain anonymous, said that the officials were probing letters sent to television show hosts Jon Stewart and Stephen Colbert, warning biological attacks on senators. The letters sent to news organizations didn't contain the powdery substance.

FBI spokesperson Chris Allen in Washington said that the federal agency was aware of the situation and was responding accordingly.

Media offices which received the letters include The Colbert Report, The Washington Post, USA Today, NPR, The Chicago Tribune, The Los Angeles Times, The Seattle Post-Intelligencer and Fox News.