A second test of the anthrax-laced letter sent to Senate Majority Leader Tom Daschle also indicates the presence a troubling chemical additive, sources tell ABCNEWS.
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Four well-placed and separate sources told ABCNEWS that initial tests also detected bentonite, though White House said initially the chemical was not found.
The first battery of tests, conducted at Ft. Detrick, Md., and elsewhere, discovered the anthrax spores were treated with bentonite, a substance that keeps the tiny particles floating in the air by preventing them from sticking together. The easier the particles are to inhale, the more deadly they are.
As far as is known, only one country, Iraq, has used bentonite to produce biological weapons, though officials caution that the presence of the chemical does not constitute firm evidence of Iraqi involvement.
White House spokesman Ari Fleischer had denied bentonite was found on the letters, but another senior White House official backed off Fleischer's comments, saying "at this point" there does not appear to be bentonite.
The official said the Ft. Detrick findings represented an "opinionated analysis," that three other labs are conducting tests, and that one of those labs had contradicted the bentonite finding. But, the official added, "tests continue."
Fleischer added that no test or analysis has concluded that bentonite is present in the Daschle anthrax, and "no other finding contradicts or calls into question" that conclusion.
Reading from what he said was a sentence from the report prepared by scientists at Fort Detrick, he told ABCNEWS, "It is interesting to note there is no evidence of aluminum in the sample." Aluminum, Fleischer said, would also be present if bentonite was.
Defense Secretary Donald Rumsfeld said that the anthrax letters were being carefully investigated, but did not confirm or deny the presence of bentonite in the Daschle letter.
While it's possible countries other than Iraq may be using the additive, it is a trademark of Iraqi leader Saddam Hussein's biological weapons program.
"It means to me that Iraq becomes the prime suspect as the source of the anthrax used in these letters," former U.N. weapons inspector Timothy Trevan told ABCNEWS.
In the process of destroying much of Iraq's biological arsenal, U.N. teams first discovered Iraq was using bentonite, which is found in soil around the world, including the United States and Iraq.
"That discovery was proof positive of how they were using bentonite to make small particles," former U.N. weapons inspector Richard Spertzel told ABCNEWS.
But officials cautioned that even if Iraq or renegade Iraqi scientists prove to be the source, it's a separate issue from who actually sent the anthrax through the mail.
"What you have to keep in mind is the difference between knowledge about what type of information you have to have to produce it, and who could have sent it," Fleischer said. "They are totally separate topics that could involve totally separate people. It could be the same person or people. It could be totally different people. The information does not apply to who sent it."