The 400 kg of HEU was largely stored in underground tunnels near the Isfahan Uranium Conversion Facility. Despite extensive Israeli and US attacks the facility, there does not seem to have been any effort to destroy these tunnels or the material that was in them. 3/17

In our investigation, we interviewed more than 200 people, obtained troves of documents and filed suit against the FDA after the agency said it would take years to turn over public records. More on how we conducted our reporting, which spanned more than a year: