A massive explosion and fire at a highly sensitive Iranian nuclear facility last week was likely an act of sabotage, intelligence officials and weapons experts said Monday, but analysts were divided over the severity of the damage to Iran’s nuclear program.

Satellite photos released over the weekend show a gaping hole in a large industrial building where Iranian technicians assembled machines that make enriched uranium. The building, on the grounds of Iran’s sprawling Natanz uranium enrichment complex, was rocked early Thursday by a mysterious blast that Iranian officials acknowledge caused “significant” damage.

A spokesman for Iran’s nuclear energy agency on Monday said the country’s security agencies knew the cause of the explosion but would provide no details for now, citing “security considerations.” Other Iranian officials have publicly suggested that the United States or Israeli operatives were to blame, although neither country has acknowledged any involvement in the incident.

A Middle Eastern security official said in an interview that the damage was caused by a “huge explosive device” planted by Israeli operatives to “send a signal” to Tehran.