The offer came as the anthrax attacks struck a third broadcast network and a New Jersey postal facility.
The cases were the fifth and sixth anthrax infections reported in the broadening wave of bioterrorism. The news only increased the pressure on investigators to track down those responsible.
"A limited number of individuals have been exposed, but our nation is quite clearly concerned," said FBI Director Robert S. Mueller III. Agents in New York, New Jersey, Washington and Florida are following "each and every lead that could disclose the identity and provide the proof against those who were responsible for these anthrax attacks," he said.
It is still not clear whether the attacks in Florida, New York and Washington came from a single source. Investigators have said the FBI has "substantive leads" in the case, but Mueller would say only that the bureau has received hundreds of leads in recent days.
"We are not in a position to determine those who are responsible," Mueller said at news briefings.
The New Jersey mail carrier's infection could be the break investigators need to narrow their search for the source of the contaminated letters mailed to NBC and Daschle.
Postal authorities said the letter carrier works a route based at a post office in West Trenton - one of 46 local post offices that send mail to the regional facility outside Trenton where the contaminated letters were postmarked. Investigators, who have fanned out across the area, can check the homes and businesses or mailboxes on her route to see if they are linked to the letters.
FBI agents have been checking sites where anthrax or the sophisticated equipment to produce it might be found. At Princeton University, a 20-minute drive from Trenton, university spokeswoman Marilyn Marks said FBI agents spoke with researchers.
"The thrust of their questions was, 'Were we doing research on campus that used anthrax?'" said Marks. "The answer is no."
Other investigators questioned pharmacists, asking whether they recalled anyone buying the large amount of the antibiotic Cipro needed to treat anthrax before the first tainted letter was mailed on Sept. 16. The amount would far exceed what is usually needed to treat more routine infections.
Attorney General John Ashcroft said investigators had not ruled out the possibility that international or domestic terrorists could be behind the anthrax scare.
"It might well be that we have opportunists in the United States or terrorists in the United States who are acting in ways that are unrelated," he said.
Authorities also were investigating whether some of the confirmed anthrax cases, and the widespread hoaxes, could be related - part of a broad plan to disrupt and distract federal officials. Medical sleuths continued yesterday to study the anthrax samples recovered from Florida, New York and Washington. The federal Centers for Disease Control and Prevention in Atlanta has said the strains from the Brokaw letter and from the Florida incident appear to match.
CDC epidemiologist Dr. David Fleming said it was not yet clear whether the Daschle sample belongs to the same strain.
But he stressed that there was no evidence that the anthrax in the Daschle letter was any more virulent, or capable of causing serious illness, than those from New York or Florida. In any case, all three samples respond well to antibiotics, authorities say, suggesting that they were not bioengineered to be resistant to the lifesaving drugs.
Since the first, fatal anthrax case - involving supermarket tabloid photo editor Robert Stevens - was made public Oct. 4, putting public health authorities on alert, none of the subsequent victims has died.
A co-worker remains hospitalized with the same inhaled form of the disease. An assistant to NBC Nightly News anchor Tom Brokaw, and later the infant son of an ABC News producer, both in New York, later were diagnosed with anthrax skin infections.
Government scientists are awaiting test results on at least four more people who may also be infected.
"We do have other individuals who are reporting skin lesions or other exposure circumstances that are under active investigation," said Dr. Julie Gerberding of the CDC. They are linked to the Florida and New York exposures, she said.
Yesterday in New York, a female assistant to CBS News anchor Dan Rather learned that a blemish on her cheek, which she first assumed to be an infected bug bite, had tested positive for anthrax in a biopsy.
The unnamed woman, whose job includes handling Rather's mail, was being treated with antibiotics and was said to be recovering. Rather said she never even took a sick day.
CBS News President Andrew Heyward said, "She's doing fine, she feels great."
Although his assistant had the primary responsibility for opening his mail, Rather said "she has no memory of anything in the mail that raised any suspicion whatsoever."
"We make no assumptions about how she contracted this," Rather said. Police, however, are operating on the theory that the anthrax entered the building in the mail.
Heyward said that because no one else at CBS has shown any symptoms, only Rather's assistant would need to take antibiotics. The CBS offices were open and operating normally, although mail security procedures had been tightened.
Dr. Stephen Ostroff of the CDC, said: "The risk currently in this building is essentially negligible." Ostroff said the epidemiological investigation at CBS is continuing. But so far, "the pattern here appears to be essentially identical" to those at NBC and ABC.
In each news office, a single case of anthrax skin infection resulted from apparent contact with anthrax spores. The only anthrax-containing letter recovered in New York, however, was the one at NBC addressed to Brokaw. That letter was disguised as a letter from schoolchildren, and was postmarked in Trenton on Sept. 16.
Acting New Jersey Gov. Donald T. DiFrancesco announced yesterday that the mail carrier had tested positive for the skin form of anthrax. A second employee - a maintenance worker at the regional center - was being tested for suspected exposure to the bacteria.
Both were being treated with antibiotics, and "both are doing well," DiFrancesco said. No other anthrax cases have been reported at the facility.
In Washington, after several days that produced sometimes-conflicting information from government officials, Bush administration officials tried yesterday to reassure a nervous city and nation by showing a united front.
"There has been some concern about mixed messages," said the newly appointed homeland security director, Tom Ridge. The former Pennsylvania governor held a morning news conference with officials from across the federal government, including the Justice and Defense departments, to signal that the situation is under control.
He said authorities are taking broad steps to protect an unnerved nation.
"We are more vigilant today, and continue to get more vigilant every day as we assess different risks that might present themselves in this country," he said.
The Postal Service announced that it was sending postcards to every U.S. household offering tips about how to handle and screen suspicious mail. "We believe the mail is safe," said Postmaster General Jack Potter.
On Capitol Hill, officials reported no new cases of anthrax exposure beyond the 31 announced Wednesday. Deputy Surgeon General Kenneth Moritsugu said environmental sweeps were under way in the Capitol and in the House and Senate office buildings. No results were available yesterday.
The House of Representatives was shuttered, and nearby congressional office buildings were dark as investigators searched for suspect mail or traces of anthrax. Senators, who had vowed to keep their chamber open, scrambled to find places to work with their offices closed.
House leaders defended their decision to shut down a day before the scheduled weekend break, even as federal health officials were saying that the contaminated areas on Capitol Hill appeared to be confined to two floors, in one wing of the Hart Senate office building.
"People have asked me all day, 'Well, what message did this send to the terrorists?'" said House Minority Leader Richard A. Gephardt, a Missouri Democrat. "Well, what message would it send to the terrorists if we stupidly put people back in harm's way, to be infected by anthrax? That hardly, to me, is an intelligent response."
Ashcroft said federal prosecutors are moving aggressively against malicious pranksters. Five people have been charged with making threats or lying to federal agents. Ashcroft said the hoaxes are wasting public resources and creating "illegitimate alarm at a time of legitimate concern." Since Oct. 1, the FBI has responded to more than 3,300 threats involving chemical or biological agents - 10 times the normal level.
Yesterday, authorities in Kenya, in East Africa, said that white powder found in a letter mailed from Atlanta had tested positive for anthrax. Two other suspicious envelopes - one from Pakistan and the other mailed within Kenya - were being tested. Officials said the first letter was mailed to a Nairobi businessman three days before the Sept. 11 attacks. It arrived in Kenya on Oct. 9, and was opened two days later.
Wire reports contributed to this article.