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“The scientific link between the letter material and flask number RMR-1029 is not as conclusive as stated” by the Justice Department, the report says. The committee and investigators agreed, though, that spores from the flask would have required one or more intermediary growth steps to become the material in the letters.

The report reveals that the FBI pursued a possible al Qaeda link to the mailings by trying without success to grow anthrax from swabs and swipes taken from an unspecified overseas site at which a terrorist group’s anthrax program was allegedly located. The samples tested positive for Ames anthrax — false positives aren’t unusual — but wouldn’t grow spores, according to sketchy information in a newly declassified document that the FBI gave the committee December or January.

The committee said the methods used in the inconclusive tests should be explored in more detail.

AP reporters Pete Yost and Matt Apuzzo in Washington contributed to this report