Iran's nuclear body has said that a fire last month at a major nuclear facility was caused by sabotage.
But the Atomic Energy Organisation of Iran (AEOI) did not say who it believed was behind the incident at the Natanz uranium enrichment site.
Some Iranian officials have previously said the fire might have been the result of cyber sabotage.
There were several fires and explosions at power facilities and other sites in the weeks surrounding the incident.
Behrouz Kamalvandi, a AEOI spokesperson, told state TV channel al-Alam on Sunday that "security authorities will reveal in due time the reason behind the [Natanz] blast".
The fire hit a central centrifuge assembly workshop. Centrifuges are needed to produce enriched uranium, which can be used to make reactor fuel but also material for nuclear weapons.
Mr Kamalvandi said last month that Iran would replace the damaged building with more advanced equipment, but acknowledged that the fire could slow down the development and production of advanced centrifuges "in the medium term".
An article by Iran's state news agency Irna previously addressed the possibility of sabotage by adversaries such as the United States and Israel, but did not accuse either of the countries directly.
Why is Natanz significant?
Natanz, about 250km (150 miles) south of the capital Tehran, is Iran's largest uranium enrichment facility.
Earlier this month, Bloomberg published details of a report by the the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, which concluded that Iran was attempting to boost uranium enrichment at the plant.
If true, the move would be in violation of a 2015 nuclear deal Iran signed with several world powers.
As part of the deal, Iran agreed only to produce low-enriched uranium, which has a 3-4% concentration of U-235 isotopes and can be used to produce fuel for nuclear power plants. Weapons-grade uranium is 90% enriched or more.
Iran also agreed to install no more than 5,060 of the oldest and least efficient centrifuges at Natanz until 2026, and not to carry out any enrichment at its other underground facility, Fordo, until 2031.
In exchange for concessions to it's nuclear programme, Iran was granted relief from international sanctions.
But last year, Iran began rolling back its commitments after US President Donald Trump abandoned the nuclear accord and reinstated crippling economic sanctions.
In November, Iran said it had doubled the number of advanced centrifuges being operated at Natanz and had begun injecting uranium hexafluoride gas into centrifuges at Fordo.
Natanz is one of several facilities being monitored by the IAEA to ensure Iran's compliance with the 2015 deal.
The IAEA's new director general Rafael Grossi has said he will visit Tehran on Monday to request access to two suspected former nuclear sites.
The IAEA has criticised Iran for not answering questions about possible undeclared nuclear material and nuclear-related activities at these sites in the early 2000s.
Iran has previously insisted its nuclear programme is not intended for military use. Officials have also denied that Mr Grossi's visit is related to moves by the United States at the UN Security Council to reimpose international sanctions on Tehran, state media reported.