"The U.S. has been observing and analyzing this facility for several years. Gradually a picture emerged of Iran's activities at and plans for the site," a counter-proliferation official said on condition of anonymity.
"This facility appears too small to produce fuel for a nuclear power reactor. The site is intended to hold approximately 3,000 centrifuges. Civilian plants typically require a far larger number," the official added.
Civilian enrichment facilities typically need tens of thousands of centrifuges to produce viable amounts of fuel for the regular operation of nuclear power plants.
(Reporting by Adam Entous; Editing by Jackie Frank)