The International Atomic Energy Agency (IAEA) reported on May 31 that Iran had failed to explain traces of uranium discovered by the IAEA months earlier at three undeclared nuclear sites in the country. In a separate report, the IAEA said that Iran's stock of enriched uranium had reached 3,241 kilograms, an increase of 273 kilograms from the previous quarter. The IAEA estimated that Iran had produced 2.4 kilograms of uranium enriched to 60 percent fissile purity and 62.8 kilograms of uranium enriched to 20 percent purity. The second report also noted that Iran was using 20 cascades of centrifuges to enrich uranium at its Natanz nuclear facility as of May 24; according to an unnamed senior diplomat, Iran had been using between 35 and 37 cascades for that purpose before an explosion at Natanz in April that Iran blamed on Israel. The IAEA added that Iran had produced 2.4 kilograms of uranium metal, a material with uses in nuclear weapons. Iran had produced only 3.6 grams of uranium metal when the IAEA last reported it three months earlier.