A fire at Iran’s main nuclear fuel enrichment site caused significant damage, setting back the country’s nuclear program by months, the government acknowledged on Sunday, after initially saying the destruction was minor.

A Middle Eastern intelligence official with knowledge of the episode said Israel was responsible for the attack on the Natanz nuclear complex on Thursday, using a powerful bomb. A member of Iran’s Revolutionary Guard who was briefed on the matter also said an explosive was used.

Both officials spoke on condition of anonymity when discussing sensitive intelligence and operational topics.

Suspicion in Iran has focused on Israel and the United States, which have sabotaged the nuclear program in the past and have vowed to keep Iran from developing nuclear weapons. In the past, Israel and the United States have used cyberattacks to damage Iran’s nuclear program, but that has been ruled out as a cause in this case, the Revolutionary Guard member said.

Israeli officials were vague on Sunday when asked about the possibility of involvement in an attack on Natanz, though they stressed the danger of a nuclear-armed Iran.

“Everyone can suspect us in everything and all the time, but I don’t think that’s correct,” Defense Minister Benny Gantz said on Sunday in a radio interview with Kan, the state broadcaster.

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“Not every event that happens in Iran is necessarily related to us,” he added, while not denying involvement.

Foreign Minister Gabi Ashkenazi, at a conference on Sunday held by the newspapers Maariv and The Jerusalem Post, said, “Iran cannot be allowed to have nuclear capabilities.” To that end, he said, “we take actions that are better left unsaid.”

Iranian officials have said publicly that they know what caused the damage at Natanz, but that they are withholding the information for now.

While investigators have considered the possibility that Natanz was hit on Thursday by a cruise missile or a drone, they view it as more likely that someone carried a bomb into the building, the Revolutionary Guard member said. They do not yet know how or when the explosives were sneaked in, but the attack clearly demonstrated a hole in the facility’s security, he said.