Senator Daschle got the first Washington letter
The suspect letter, addressed to Democratic Senator Patrick Leahy of Vermont, was dated 9 October and postmarked from Trenton, New Jersey - the same as an anthrax-contaminated letter sent to Senate leader Tom Daschle.
The building where Mr Leahy works and other Senate offices were ordered shut Saturday while tests are carried out for traces of anthrax on the premises.
Mr Leahy said he hoped the new letter might shed light on who is carrying out the anthrax mail campaign.
"We have a lot of very good men and women in that office, this is not the sort of thing they signed on for," he said.
The letter for Mr Leahy was found by investigators who have been hunting through quarantined, unopened mail since Mr Daschle's letter was discovered.
"There's been concern all along that there was a possibility that we, in fact, had a second letter somewhere in the congressional mail system. That letter has now been found," said US Capitol Police spokesman Lieutenant Dan Nichols.
No new cases
No new cases of anthrax infection have been reported in the US for more than two weeks.
Four people have died and 13 more tested positive for anthrax since the beginning of October, amid fears of biological attack on the US following the 11 September suicide hijackings.
If confirmed, the latest letter will be only the second to be discovered in Washington. Two others were sent to the New York Post newspaper and the New York offices of media network NBC.
Traces of anthrax were found in the offices of three senators - Dianne Feinstein of California, Larry Craig of Idaho and Bob Graham of Florida - last week.
All have offices in the same building as Mr Daschle, which has to be decontaminated with chlorine dioxide gas.
Mr Leahy's office is in a different building, but it is not clear where the letter was when deliveries to Congress were suspended on 15 October.
Psychological profile
The FBI indicated last week that it believes the outbreak originated at home rather than abroad, and may be the work of a single person.
Osama Bin Laden, the prime suspect in the 11 September attacks, has said he knew nothing of the outbreak.
Meanwhile FBI agents say they have built up a psychological and linguistic profile of the culprit, who is probably an adult male with no more scientific knowledge than a lab technician.
His equipment need not have cost more than $2,500 and he could have set it up in his garage, or in his attic.
He is, agents believe, a loner, rational and methodical but "lacking the personal skills necessary to confront others".