Washington ? Looming war with Iraq and growing concern over the threat of bioterrorism are moving federal officials to consider what was once unthinkable: offer the risky smallpox vaccine to the general public before an attack ever occurs.

Just three months ago, federal advisers were recommending that only select hospital workers get the smallpox vaccine, maybe 20,000 total.

Now, the Bush administration is preparing to offer it to all 280 million Americans. The question being debated is not whether the general public should get it, but how fast and under what circumstances, according to three officials involved in the planning.

Experts don’t know if the nation will ever be attacked with smallpox, which kills one-third of its victims. Eradicated from nature two decades ago, it is still feared as a bioterror agent. But the vaccine itself carries rare but serious risks, including death, complicating any decision to inoculate people absent a certain risk.

The Bush administration has yet to make final decisions or announce plans for what is called “pre-attack” smallpox vaccination. But administration officials say the consensus is to begin vaccinating those at greatest risk of encountering a highly contagious smallpox patient, such as hospital emergency room workers. That could total a half-million people. Then the vaccine would be offered to nonhospital health workers, such as primary-care doctors, and to police, firefighters and other emergency workers.

At some point after that, it would be offered to the general public.

“You start with one group and based on their potential risk, you keep expanding,” one administration official said Thursday, speaking on condition of anonymity.

But it hasn’t been decided how many people will get vaccinated during the first wave of shots, or how long it will take to offer them to everyone.

At issue, officials say, are important details such as who should get the vaccine while it’s still an experimental drug, which requires a lengthy procedure to ensure that people understand and accept the risks.