Senator Daschle received an anthrax-laced letter
Experts say the quantity is so small that it does not pose a serious health risk, but plans to reopen the Hart Senate Office Building on Tuesday have been put on hold because of the discovery.
"He laughed and said we don't know anything about it," said journalist Hamid Mir, who interviewed Bin Laden in Afghanistan on Thursday.
The FBI has indicated that it believes the outbreak originated at home rather than abroad, and may be the work of a single person.
Officemates
The three senators - Dianne Feinstein of California, Larry Craig of Idaho and Bob Graham of Florida - all have offices in the same building as Senate majority leader Tom Daschle, who received an anthrax-laced letter last month.
Nearly 30 congressional workers are known to have been exposed to the bacteria, but none have become ill.
At least three letters containing anthrax spores have been sent since the 11 September attacks on New York and Washington. Four people have died, two of them postal workers.
Four New Jersey post offices have been disinfected and are open for business after anthrax was discovered.
Two post offices in Colorado, meanwhile, have been ordered to close while authorities try to confirm that a worker's illness is not anthrax.
Profile of suspect
President George W Bush has ordered increased security at airports for the holiday season, which begins with Thanksgiving on 22 November.
FBI agents say they have built up a psychological and linguistic profile of the culprit.
The FBI say he is probably an adult male who may have 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".
BBC Washington correspondent Tim Franks says the FBI hopes that its detailed analysis of the culprit will prompt a friend or relative to tip them off.