"The evidence that has been put forward so far demonstrates clearly that Iran has been moving in the direction of creating a nuclear weapon," Powell said. "And that is why the International Atomic Energy Agency [IAEA] got so involved, why the Russians have been careful about providing fuel for the new reactor at Bushehr, and why the European Union sent their three foreign ministers in to get the Iranians to stop."
But Iranian officials, including President Hojatoleslam Mohammad Khatami, say Tehran is only interested in nuclear energy for peaceful purposes.
"We have made our choice: Peaceful nuclear technology -- yes. Atomic weapons -- no. Not 'no' only for ourselves -- no [nuclear weapons] for the region, no [nuclear weapons] for the world," Khatami said.
So who is right?
Analysts say the only way to decide is to weigh the physical evidence that has kept the crisis at the center of the world stage since 2002. Much of that evidence emerged when an exiled Iranian opposition group exposed a secret pilot project to master the process of uranium enrichment. The project included some 160 assembled gas centrifuges -- plus equipment to build some 5,000 more -- hidden in reinforced underground bunkers strong enough to resist air strikes.
In the process, uranium is first converted to uranium hexafluoride gas, a substance that is fed into centrifuges used to enrich uranium.
The discovery of the sites was alarming because enriched uranium can be used either as a nuclear fuel or -- at higher levels of enrichment -- as material for nuclear bombs. It also showed that Iran was violating safeguards in the international Nuclear Non-Proliferation Treaty (NPT), of which it is a signatory. The treaty gives Tehran the right to acquire nuclear energy for peaceful purposes, but also binds it to declare all such facilities to the UN's IAEA and to open such sites to its inspectors.
Later visits to the site by IAEA inspectors revealed that some of the centrifuges had been used to enrich two types of uranium to 20 percent or more. That is far higher than the usual 2 to 3 percent enrichment level required for nuclear fuel.
Nonproliferation experts say uranium enriched to a 20 percent level is sufficient to make a very cumbersome nuclear bomb. But it falls well short of the enrichment levels -- 90 percent or higher -- needed to produce the kinds of missile or airplane-deliverable warheads that make a country a nuclear power.
Fred Wehling, an arms-control expert at the Center for Nonproliferation Studies in Monterey, California, says the discovery of Iran's uranium-enrichment activities made many nonproliferation experts skeptical of Tehran's explanation that it was seeking only to master the nuclear fuel cycle for energy purposes.
"If Iran was to develop an indigenous enrichment capacity, it could eventually make its own fuel, which could then be used in Bushehr," Wehling said. "But then if that were really the case, you wouldn't need to go to all the trouble of having a clandestine facility and acquiring uranium under the table to test it and so on."
Equally worrisome, nonproliferation experts said, are indications that Iran might have built some of its uranium-enrichment equipment according to blueprints acquired on the global black market for nuclear secrets.
The suspected source is the trafficking network organized by Pakistani nuclear scientist Abdul Qadeer Khan. It is not known whether the network also sold Iran information about how to design a nuclear weapon, as it did to Libya.
Since the discovery of Iran's clandestine efforts, Tehran has sought to assure the IAEA that it is now fully cooperating with international inspectors to disclose all of its nuclear work.
But Tehran said it still insists on its right under the NPT to develop its own nuclear fuel cycle and will not give that up.
There are varying estimates of how long it could take Iran to develop a nuclear weapon, if it wished.
Daniel Keohane, an international security expert at the Center for European Reform in London, put the timeline this way: "If you ask the Europeans how far away are the Iranians from a bomb, the general consensus seems to be four to six years. And in Washington, I understand, the consensus is closer to three years and possibly even sooner, depending on how the Iranians behave over the next year or so."
Keohane said any progress Tehran might make in developing a nuclear weapon will be determined by how much it cooperates with current efforts by European states to persuade it to give up programs related to uranium enrichment in exchange for trade incentives.