Asked why they're suspicious of Iran's nuclear intentions, US officials point to Natanz.
Iran's Natanz nuclear site is in a remote area 200 miles south of Tehran. Key facilities are buried, with vehicle entrance ramps hidden beneath dummy buildings. Construction there has continued in recent months despite Iran's nuclear negotiations with the West - recent satellite photos revealed at least seven new buildings.
Iran's leaders have long said they are conducting nuclear research for peaceful purposes. They claim they want only to learn how to produce fissionable fuel for power plants, as they're allowed to do under terms of the Nuclear Non-Proliferation Treaty.
That explanation makes no sense for a nation with 10 percent of the world's known oil reserves, US officials and some outside experts say. They claim that the concrete and steel of Iran's nuclear infrastructure shows Tehran's true intentions.
"It's going up in front of our eyes," says David Albright, president of the Institute for Science and International Security (ISIS), of the Natanz facility.
On Thursday the US repeated that neither it nor its European partners want to return to the negotiating table with Iran. The international community is united in mistrusting Tehran with nuclear technology, said US Secretary of State Condoleezza Rice. "We have been very clear that the time has come for a referral of Iran to the [UN] Security Council," Secretary Rice told reporters before a meeting with South Korea's foreign minister.
Rice's choice of the word "referral" was deliberate. If Iran is only "reported" to the Security Council, debate might lack legal weight. A formal "referral" is necessary if the Council is to impose any penalty, such as economic sanctions.
Iranian officials, for their part, insist that their nuclear program is a peaceful one. But they have remained defiant as international criticism has grown.
On Monday, Iran's representative to the International Atomic Energy Agency (IAEA), Ali Asghar Soltanieh, said Tehran would forge ahead with a full-scale uranium enrichment program if it is referred to the UN Security Council. Such a referral would be a "hasty decision", he told the Associated Press.
Iran's nuclear program began when the Shah purchased a research reactor from the US in 1959.
The Shah had big plans for a network of 23 power reactors, but the US did not consider this a danger, because he was an ally, and he did not ask for technologies to enrich or reprocess spent nuclear fuel.
In the years after the Iranian revolution, US concern about Iran's nuclear efforts focused on Russian help on the Bushehr nuclear reactor project. Those concerns were heightened in 2002, when a dissident Iranian group, the National Council of Resistance of Iran, helped expose clandestine nuclear facilities at Natanz and Arak.
This exposure forced Iran to declare the facilities to the IAEA, as Tehran is a signatory of the Non-Proliferation Treaty. It was not just the secrecy that set off alarms in the US, but the nature of the newly revealed sites - and Natanz, in particular.
The 100,000-square-meter Natanz facility is the location of a pilot uranium centrifuge enrichment plant, as well as a future commercial-size centrifuge plant. If Iran masters enrichment technology, it will be able to make its own fissile material, including possible bomb material - and it will have jumped the most difficult hurdle on the path to becoming an atomic power.
On Jan. 10, Iran removed IAEA seals placed at Natanz and other facilities to verify a suspension of work. Centrifuges, which enrich a gas form of uranium by spinning at incredibly high speeds, are difficult to run, and Iran may have trouble restarting its pilot centrifuge plant.
"They're not quite junk, but it's going to take a lot of effort to get them up and running again," says Charles Ferguson, a nuclear science and technology expert at the Council on Foreign Relations.
But once the centrifuges are running, the clock will be ticking on Western estimates of when Iran might have enough fissile material for a nuclear device. Israeli intelligence argues that Iran is only two years away from a bomb, but others think it will take more time.
On Saturday, Israel repeated that it would not accept a nuclear Iran under any circumstances. Iran said Sunday that Israel would be making a "fatal mistake" if it takes military action against Tehran's nuclear program and dismissed threats from the Jewish state as a "childish game."
The ISIS - which last week released the satellite photo of Natanz - says Iran could have its first nuclear weapon by 2009.
But why won't Iran just use the centrifuges to produce low-enriched uranium for power reactors? That's what Tehran says it wants to do.
US intelligence rejoins that Iran hid the plants from the IAEA - a suspicious act. Nor has Tehran told the full story of where its centrifuge technology came from, although most experts think it is Pakistan.
Furthermore, Iran's investment in the nuclear-fuel cycle makes no sense from a civilian viewpoint, the US says. Iran lacks adequate deposits of natural uranium to ever be self-sufficient in civil nuclear power, according to a Department of Energy analysis in 2005. The DOE says that Iran's nuclear infrastructure is about the right size for weapons capability, as seen when it is compared with the program of another nation, presumably Pakistan.
"It is difficult to escape the conclusion that Iran is pursuing nuclear weapons," says the DOE intelligence analysis.