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.

Read more: Washington Post