Iran's Atomic Energy Organization said on Sunday that "sabotage" was the cause of an explosion that damaged the Natanz nuclear facility last month.
"Security investigations confirm this was sabotage and what is certain is that an explosion took place in Natanz," said spokesman Behrouz Kamalvandi.
"But how this explosion took place and with what materials... will be announced by security officials in due course," he was quoted as saying by state news agency IRNA.
Iran said after the July 2 incident that it had determined its cause but declined to release details due to "security concerns".
Natanz governor Ramezan-Ali Ferdowsi said a fire had broken out at the site, but the country's atomic agency said it caused no casualties or radioactive pollution.
At the time, IRNA published an editorial warning Iran's arch-foes against hostile actions, saying unnamed Israeli social media accounts had claimed the Jewish state was behind it.
The incident came six days after an explosion near a military complex rocked Tehran.
The blast in the Parchin area southeast of the Iranian capital was due to "leaking gas tanks", the defense ministry said.
The Islamic republic resumed uranium enrichment at the Natanz complex, in central Iran, in September last year.
The move came after the United States unilaterally withdrew in 2018 from an international accord that gave Iran relief from sanctions in exchange for curbs on its nuclear program.
Tehran has always denied its nuclear program has any military dimension.