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US government officials concluded that particles of uranium found at Iran’s Parchin military base and revealed in the International Atomic Energy Agency’s final report on the country’s past nuclear activities were likely tied to the regime’s nuclear weapons program. The admission further underscores concerns that the IAEA’s investigation into Iran’s nuclear activities at Parchin should not have been closed following the report’s publication, Homeland Security News Wire reports.

“The man-made uranium found at Parchin, which has only low-levels of fissionable isotopes, can be used as a substitute for weapons-grade materials in developing atomic bombs, according to nuclear experts,” the Wall Street Journal reports. “It can also be used as component in a neutron initiator, a triggering device for a nuclear weapon.”

“The existence of two particles of uranium there would be consistent with our understanding of the involvement of Parchin in a past weapons program, but by themselves don’t definitively prove anything,” said a senior government official. However, the administration believes it has other information that confirms there was weapons-related activity there.

Critics of the nuclear deal with Iran have asserted that the uranium found at Parchin showed that the “Obama administration didn’t go far enough in demanding Iran answer all questions concerning its past nuclear work before lifting international sanctions in January,” the Journal wrote. They also questioned whether the IAEA can effectively monitor Iran without fully understanding the country’s past nuclear work.

Determining what research Iran carried out at Parchin is complicated by a controversial provision in the nuclear deal that allowed Iran to self-inspect the facility. Iran has also carried out significant construction at the facility, compromising evidence of past work there.

Independent experts argued when the IAEA report was released last December that the agency should not close its investigation of Iran’s past nuclear work in light of the discovery of uranium particles at Parchin. The report also found that Iran was developing a nuclear weapon until 2009, contradicting U.S. estimates that Iran halted its efforts to develop a nuclear bomb by 2003.