There are three main reasons the secret site was built to manufacture fuel for nuclear bombs, U.S. officials said.
It was clear from satellite imagery that a facility was being constructed underground and heavily disguised. Also, it was built on a military base and protected by armed guards 24 hours a day. And intelligence information shows that it is designed to hold only about 3,000 centrifuges to enrich uranium -- not enough to fuel a civilian power plant, which would need around 50,000 centrifuges, but enough to make highly enriched uranium for a nuclear weapon.
After the tests Sunday, the head of Iran's Revolutionary Guard Air Force said they were a success, and the country was ready to fight off attacks from any country.
"We are going to respond to any military action in a crushing manner and it doesn't make any difference which country or regime has launched the aggression," state media quoted Gen. Hossein Salami as saying.
Iran insisted that the facility is part of a peaceful nuclear energy program but has not explained why the site was hidden and heavily guarded, and why it was not immediately reported to international regulators.
"For the Iranians themselves, this must be incredibly unsettling for them to know that Western intelligence agencies have penetrated state secrets," said Karim Sadjadpour, an Iranian expert at the Carnegie Endowment for International Peace.
But attacking Iran's nuclear site could open up a new can of worms. Not only would it create backlash against the United States, but according to a U.S. intelligence official, the Iranians have a decentralized system of nuclear sites all over the country, with no "single point of failure."
So if one site were taken out, others would still exist, which is a problem U.S. officials may be addressing for many more years to come.
ABC News' Huma Khan contributed to this report.