Both envelopes were postmarked in Trenton, N.J., and the block handwriting on the outside of each envelope was similar.
Despite the similarities, federal authorities say the anthrax sent to NBC is very crude compared to the weapons-grade bacteria mailed to Daschle's office.
• Feds: 'No Direct Link' to Bin Laden
Senior law enforcement officials tell ABCNEWS investigators now believe that the Daschle and Brokaw letters are linked, but were still looking for connections to the Sept. 11 attacks.
"While we have not ruled out linkage to the terrorist attack of Sept. 11 or the perpetrators of that attack, we do not have conclusive evidence … that it is part of that endeavor," Attorney General John Ashcroft told reporters at the Justice Department.
FBI Director Robert Mueller said investigators had so far found "no direct link" between the anthrax cases and "organized terrorism."
President Bush said a day earlier "there may be some possible link" between the cases and terrorist mastermind Osama bin Laden.
• ABC Involved in Anthrax Case
A 7-month-old infant, who was in ABCNEWS headquarters for several hours on Sept. 28, was diagnosed with anthrax late Monday — one of four cases of infection involving a major American media outlet this month.
The child, the son of a news producer, developed a rash a day after the visit and has tested positive for cutaneous or "skin" anthrax, the least harmful form of the disease. Cutaneous anthrax is highly treatable.
"The baby showed symptoms within about a day — they were unusual symptoms that appeared to be related to an infection," ABCNEWS President David Westin said today on the network's Good Morning America. "The hospital … had some difficulty diagnosing this, thought at first it was a spider bite."
Westin reported the child was responding well to treatment and his prognosis was excellent.
New York police investigators today completed taking environmental samples from the areas of ABCNEWS building they believe the child was on Sept. 28, Westin said in an e-mail to employees. A helicopter flew the samples to Albany where they are being tested at a New York State lab.
So far, more 200 ABCNEWS employees have been questioned by health and law enforcement officers, the Westin e-mail said. Health officials also began the nasal swab testing of approximately 100 ABCNEWS employees who reported either having contact with the child or being in the suspected affected areas of building.
The ABCNEWS case was revealed Monday, the same day a worker at American Media Inc. in Boca Raton, Fla., was diagnosed with the usually fatal inhalant form of anthrax. Another employee at AMI, which publishes The Sun and other tabloids, died of the same disease on Oct. 5.
And an assistant to NBC News anchor Tom Brokaw at the network's New York headquarters was diagnosed Friday with cutaneous anthrax. A granular brown substance contained in a letter opened by the woman has tested positive for anthrax spores.
Environmental surveys were conducted Monday night at the New York offices of CNN, CBS, The Associated Press, The Daily News, Fox News and The New York Post as a precautionary measure.
"So far we haven't come up with anything," New York Mayor Rudolph Giuliani said on Good Morning America.
"The thing to emphasize is we're talking about a very, very small number of cases," the mayor added. "All of this is treatable and very successfully treatable."
• Crackdown on Bioterror Hoaxes