Traces of anthrax bacteria have been found in the mailrooms of two more senators.
A congressional official says it had been found in the mailrooms of Democratic senators Mr Edward Kennedy, of Massachusetts, and Mr Christopher Dodd, of Connecticut.
It's thought the bacteria got there through contact with anthrax-bearing letters sent to Senator Patrick Leahy or the Senate Majority leader Mr Tom Daschle.
So far, traces have been found in 13 senators' offices besides Mr Daschle's, whose office is the only one known to have actually received an anthrax letter.
A federal law enforcement official said a sample taken from a plastic evidence bag containing a still-unopened letter to Mr Leahy contains at least 23,000 anthrax spores, enough for more than two lethal doses.
The official, said there were three times more anthrax spores in the single sample taken from the plastic bag than in any of the other 600 bags of mail examined by the FBI before.
At the Pentagon, officials began taking new precautions against anthrax-tainted mail by requiring that all mail be opened, visually inspected, X-rayed and tested for biological or chemical materials. Once checked, mail will be held for up to three days to await test results before delivery inside the building.
AP