It was in the summer of 2001 when a top secret message was delivered to Farid Solemani (search).

“We received news from a very valuable source within the Iranian regime,” said Solemani, who worked for more than a decade to expose Iran's nuclear program.

That message, if true, was explosive in more ways than one.

It was either "the biggest breakthrough that we have had in terms of opening up the secret of the Iranian nuclear program … or it's a hoax,” said Solemani, who is a senior member of the Paris-based National Council of Resistance in Iran (NCRI) (search).

It wasn't a hoax.

His secret sources identified two sites — one in the town of Arak and one in Natanz — that were massive plants, disguised as anti-erosion facilities. Instead, the sources said, the two facilities were the heart of an advanced, clandestine nuclear weapons program.

When NCRI broke the story months later at a news conference in Washington, it was a shocking development.

“We knew back in 2000, 2001 that Iran had secret nuclear sites. And we were actually trying to find out where they were,” said David Albright (search), a former nuclear inspector for the International Atomic Energy Agency who now heads the Institute for Science and International Security think tank in Washington.

Albright went to work analyzing satellite photographs of the sites NCRI spoke about, beginning with the Arak site.

“We could look this right away and know this is a heavy water production plant just from the satellite imagery,” he said.

What he discovered at the Arak site worried him more than ever. Iran was not only building a plant — it had plans for a reactor as well.

“I mean when we first saw it, we said it was a heavy water production plant. Which implies it must be a heavy water reactor,” Albright said.

But this heavy water plant would be too big for research and too small to generate significant amounts of electricity. It would, however, produce plutonium, which is mainly used to make nuclear bombs.

Albright then turned his attention to the Natanz site.

“We actually had trouble with this,” he said. “What we had to do was really sort of do a series of interviews and triangulation to get out that this was actually a gas centrifuge plant.”

For Albright, this discovery presented a large problem because gas centrifuges are used to enrich natural uranium. Enrich it a little and you get "low-enriched uranium," which is used to fuel a nuclear reactor to generate electricity. But those centrifuges could also produce highly-enriched, or weapons-grade, uranium.

“Within a few days, they could make enough for a bomb,” Albright said.

Not just one bomb. The two massive underground facilities — each the size of a football field — are designed to hold 50,000 or more centrifuges. That could produce uranium for 25 to 50 nuclear bombs a year.

Another site that set off alarm bells was the Parchin military base (search), where Solemani's dissident group claims Iran is using as another location to enrich uranium.

Albright said he doesn't know about that, but suspects that Iran is testing the explosive triggers needed to detonate a nuclear warhead.

Until the allegations came to light, the world thought Iran had at best a rudimentary nuclear energy program.

At its center was Bushier, a half completed light-water reactor, started by the Germans and bombed by the Iraqis before the Russians resumed construction in 1995. Officials also knew about a handful of uranium mines in Sanghand and Ardekan, and a uranium ore processing facility in Isfahan.

None of it suggested the ayatollahs were close to getting a nuclear bomb.

But add in the once-secret sites in Natanz and Arak, plus access to ballistic missiles, and suddenly experts decided Iran had all it needed to turn its uranium ore into a nuclear arsenal that could target Israel, Turkey and eventually even Paris and Berlin.

Iran will have the capability in just a few years, according to the U.N.’s International Atomic Energy Agency (IAEA) (search).

Others think it could be a matter of months.

All the nuclear facilities officials do know about are perfectly legal under the international treaty that's supposed to stop the spread of nuclear weapons.

“One of the weaknesses of the nonproliferation treaty … was that it allows countries to get a screwdriver's turn away from having a nuclear weapon,” Albright said.

Iran was only obligated to disclose the facilities and allow inspectors from the IAEA to inspect them. The fact the ayatollahs hid them, according to President Bush (search), shows what they're up to.

“And remember, this all started … because somebody told on them. It was an Iranian group that brought forth the information,” Bush said at a Feb. 17 news conference. “And so you can understand our suspicions.”

For Solemani, the various pieces to the puzzle reveal that Iran is trying to develop nuclear weapons and the world needs to act “decisively.”

“If the Iranian regime, bent on exporting Islamic fundamentalism to all over the Muslim world, is armed with nuclear weapons — that is a catastrophe for the whole world, not just the United States, not just Europe — for the whole global community,” Solemani said.

But what some find disturbing is that for 18 years, Iran built its clandestine program — which went unnoticed by the international body tasked with protecting against nuclear dangers.

At the IAEA’s headquarters in Vienna, Director General Mohammad ElBaradei (search) told FOX News that he was surprised Iran had kept details secret from them for nearly two decades.

“I know we had a problem on our hands,” he said.

After Iran got caught, it responded with a series of lies and fishy explanations. But it now admits it's building a reactor, which could allow it produce plutonium without foreign help.

Iran denied it was building a heavy water reactor — one that would produce plutonium — until satellite photos proved it was lying.

IAEA inspectors proved the discovered gas centrifuges were Pakistani designs — passed on by the notorious black market nuclear arms dealer A.Q. Khan (search). Confronted with that, Iran first said it never dealt with Khan, then admitted it did.

Then there's a military facility in Lavizan, which was never declared a nuclear site by Iran.

But when the IAEA learned Iran moved advanced radiation detectors there, it asked to take a look. Iran's response: it bulldozed the place.

ElBaradei (search) said the jury is still out on whether Iran is actually trying to build nuclear weapons though he concedes Iran has “credibility gap.”

“They need to work hard, more than anybody else to restore credibility, to restore confidence,” he said.

To restore confidence, Iran did say it would halt its uranium enrichment program while the IAEA finishes its investigation and while Britain, Germany and France try to strike a deal to give Iran nuclear power, but not nuclear weapons.

Meanwhile, however, Iran is denying IAEA inspectors complete access to the Parchin military facility, where Iran is suspected of testing nuclear triggers — and perhaps running another secret enrichment operation.

But most of the staff FOX News spoke with at IAEA headquarters — from ElBaradei on down — seemed relatively confident they could keep a watch on Iran and keep it from acquiring a bomb.

ElBaradei is doing more than investigating and reporting. He's advocating that the West give a helping hand to Iran.

“You need to address security," he said. "You need to address [that] you know a country’s sense of isolation, and you need to give them incentives."

For Iranian officials, the effort made to develop a nuclear program has been worth the pain.

“It is something that we have acquired through blood, sweat and tears,” said Sirous Nasseri (search), one of Iran’s top nuclear negotiators, who spoke with FOX News in Vienna.

Asked if Iran intended to create nuclear bombs, Nasseri said: "The hell we will not. It is clear that we won’t.”

“You know that the agency [IAEA] has been scrutinizing everywhere, everything, in Iran for the last two years," he said. "They have not found a single shred of evidence that there is any diversion [from] military purposes.”

Nasseri said Iran needed to be hush-hush about what they were doing because if they had reported everything they were up to, they would have been stopped in their efforts to develop nuclear power.

“What choice do we have?” he asked.

FOX News' Chris Wallace, Jonathan Hunt and Greg Palkot contributed to this report.