Samples taken from two sites during inspections in the fall by the U.N.’s International Atomic Energy Agency contained traces of radioactive material, the diplomats said, that could indicate Iran has undertaken work on nuclear weapons, based on where it was found. The diplomats said they didn’t know the exact nature of what was found.

Last year, Iran blocked IAEA inspectors from checking the sites involved for seven months, leading to a standoff. Tehran has long denied that it has sought to make an atomic bomb and said all of its nuclear work is for peaceful purposes such as power generation and health care. There was no immediate comment from Iran on the findings…

The IAEA listed in a report in June questions it was asking Iran to clarify on a range of work that could be used for nuclear weapons. One suspicion was Iranian drilling of a uranium metal disc that could be used to create material for a neutron initiator, experts say, a key component of a nuclear weapon. A second suspicion was that nuclear material had been introduced at a site where Iran may have tested high explosives that can be used to detonate a nuclear weapon.