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.