Some Congressional offices and media organizations received threatening letters containing a suspicious powdery substance that was found to be harmless, the F.B.I. said Wednesday. The Senate sergeant-at-arms, Terrance W. Gainer, said in a memorandum to Senate offices that the letters were sent to three state and home district offices. He did not identify the offices, but a district office of Speaker John A. Boehner, Republican of Ohio, received one, a spokesman said. Letters were also sent to several media organizations.