The IAEA’s second investigation into Iran's past nuclear activities began in 2018 and is focused on evidence suggesting that Iran failed to declare all nuclear materials and activities from its pre-2003 nuclear program as legally required by its safeguards agreement. The investigation does not re-open the military dimensions file: it is focused on Iran's nuclear material accountancy obligations under its safeguards agreements.
As part of this investigation, the IAEA visited three undeclared locations in 2019 and 2020. Environmental samples taken from all three sites indicated that processed uranium had been present at all three locations. Iran has yet to provide the IAEA with technically credible explanations for the presence of uranium at two of those sites, Turquzabad and Varamin. In a May 2023 report, the IAEA said it had no further questions regarding the third site where uranium was detected, Marivan. The IAEA assessed that Iran conducted “explosive experiments with protective shielding in preparation for the use of neutron detectors.” Iran claimed that a third-party state operating a mine and chemical lab at the site contaminated the area with uranium. The IAEA said this was a "possible" explanation.