UN inspectors have found evidence of illicit nuclear activity in an Iranian warehouse which Israeli Prime Minister Benjamin Netanyahu claimed in an address to the UN last September was used to store nuclear equipment and material, four Israeli officials told me.

Why it matters: The Iranians claimed at the time that the warehouse in Tehran was a carpet factory, and denied Netanyahu's accusations that it was tied to Iran's covert military nuclear program. Storing nuclear materials secretly without reporting it to the International Atomic Energy Agency (IAEA) is a violation on the nuclear proliferation treaty to which Iran is a party.

Flashback: Netanyahu claimed in his speech that Iran had removed 15 kilograms of undeclared enriched uranium from the facility in August 2018. He said that was an attempt to "clean up" the secret site and hide their illicit activities from the IAEA.

Israel passed information about the warehouse to the IAEA, and UN inspectors visited the site several months ago, Israeli officials tell me. Their last visit was in March.

IAEA inspectors took soil samples to try and find evidence of radioactivity. The IAEA has since been analyzing the results and preparing a report.

The tests came back positive, according to the Israeli officials, and in the last few weeks it became clear that the remains of radioactive material were found at the site. The officials say that indicates Iran was storing undeclared nuclear equipment or materials.

The backdrop: The news comes ahead of an election re-run in Israel, and amid rising tensions between the U.S. and Iran. Netanyahu has been advocating for tough actions from the White House.

Yesterday, Netanyahu held his second phone call with President Trump of the past week. The call focused on Iran, and Netanyahu thanked Trump for his intention to announce new sanctions.

What's next: The officials told me Israel and the U.S. hope the IAEA will publish a report about its findings at the site and circulate it to all the member states in its board of governors soon.