Over the past several years, the United States has embarked on a program of secret research on biological weapons that, some officials say, tests the limits of the global treaty banning such weapons.
The 1972 treaty forbids nations from developing or acquiring weapons that spread disease, but it allows work on vaccines and other protective measures.
Government officials said the secret research, which mimicked the major steps a state or terrorist would take to create a biological arsenal, was aimed at understanding the threat better.
The projects, which have not been previously disclosed, were begun under President Clinton and have been embraced by the Bush administration, which intends to expand them.
Earlier this year, administration officials said, the Pentagon drew up plans to engineer genetically a potentially more potent variant of the bacterium that causes anthrax, a deadly disease ideal for germ warfare.
The experiment has been devised to assess whether the vaccine now being given to millions of American soldiers is effective against such a superbug, which was first created by Russian scientists. A Bush administration official said the National Security Council was expected to give the final go-ahead later this month.
The treaty, a Bush administration official said, allows the United States to conduct research on both microbes and germ munitions for "protective or defensive purposes."
Two other projects completed during the Clinton administration focused on the mechanics of making germ weapons.
In a program code-named Clear Vision, the CIA built and tested a model of a Soviet-designed germ bomb that agency officials feared was being sold on the international market. The CIA device lacked a fuse and other parts that would make it a working bomb, intelligence officials said.
At about the same time, Pentagon experts assembled a germ factory in the Nevada desert from commercially available materials. Pentagon officials said the project demonstrated the ease with which a terrorist or rogue nation could build a plant that could produce pounds of deadly germs.
Both the mock bomb and the factory were tested with simulants — benign substances with characteristics similar to the germs used in weapons, officials said.
Several officials who served in senior posts in the Clinton administration acknowledged that the secretive efforts were so poorly coordinated that even the White House was unaware of the full scope of the projects.