Inspectors have found such uranium, which at extreme enrichment levels can fuel bombs, twice in the past. The International Atomic Energy Agency concluded that at least some of those samples came from contaminated equipment that Iran had obtained from Pakistan.
But in this case, the nuclear fingerprint of the particles did not match the other samples, an official familiar with the inspections said, raising questions about their origin.
�We need to be very concerned that Iran may well be undertaking experiments, and may be undertaking the construction of centrifuge machines, out of sight of I.A.E.A. inspectors.�
Robert Joseph, the under secretary of state for arms control and international security
Highly enriched uranium, containing 80 percent or more of the rare uranium-235 isotope, is considered bomb grade and can be fashioned into the core of a nuclear weapon.