Trace amounts of anthrax have been found in five more offices of the Hart Senate Office Building.
Investigators say these finds may be cross-contamination from an letter sent to Senate Majority Leader Mr Tom Daschle, the US Capitol Police said.
Tests found minimal amounts of anthrax in the offices of Democratic Senators Ms Barbara Mikulski of Maryland, Mr Russ Feingold of Wisconsin, Mr Max Baucus of Montana and Mr Joseph Lieberman of Connecticut and Republican Mr Arlen Specter of Pennsylvania, a police statement said.
The Hart building has been closed since October 17th, two days after discovery of the anthrax-laced letter mailed to Mr Daschle, a South Dakota Democrat.
Investigators have not ruled out the possibility that another letter with anthrax may have been sent to Capitol Hill.
There have been 17 confirmed cases of anthrax in the US since early October, including 10 cases of inhalation anthrax. Four people, including two Washington postal workers, have died.