A man was arrested and charged in connection with about 100 envelopes containing threatening letters and a suspicious powdery substance, which were posted from Oregon to members of the US Congress.
A federal grand jury returned a two-count indictment charging 39-year-old Christopher Lee Carlson of the Portland area, with one count of mailing a threatening communication to a member of Congress and one count of mailing a letter threatening to use a biological weapon to a US senator.
Mr Carlson was arrested at his home yesterday, the US Capitol Police said in a statement after an investigation it conducted with the FBI and US Postal Inspection Service.
All the envelopes examined so far, which were postmarked in Portland and contained an unidentified white powder, have tested negative for any toxic substances, the US Attorney's Office in Portland said in a statement announcing the indictment.
But both letters cited in the indictment, one sent to House of Representatives Speaker John Boehner and the other to Senator Barbara Mikulski contained threats to the lawmakers.
The letter to Ms Mikulski, addressed to her Baltimore office, "indicated that it may contain a lethal pathogen to which the recipient may have been exposed," the indictment said.
Neither the indictment nor the press release accompanying it made any mention of a possible motive for the mass mailings of menacing letters.
The federal prosecutor's statement said Mr Carlson was expected to be arraigned in US District Court on Monday.
