PARIS — The inferno that raged through Notre Dame Cathedral for more than 12 hours destroyed its spire and its roof but spared its twin medieval bell towers, and a frantic rescue effort saved the monument’s “most precious treasures,” including the Crown of Thorns purportedly worn by Jesus, officials said Tuesday.

Also surviving was the Roman Catholic cathedral’s famous 18th century organ that boasts more than 8,000 pipes, after a plan to safeguard the masterpieces and relics was quickly put into action after the fire broke out Monday. Statues removed from the roof for restoration just days ago also were saved.

But the cathedral’s high altar was damaged by falling debris when the spire collapsed, one official said.

Authorities consider the fire an accident, possibly as a result of restoration work at the global architectural treasure that survived almost 900 years of tumultuous French history but was devastated in the blaze on the second day of Holy Week.

Paris prosecutor Remy Heitz said the inquiry into what caused the fire would be “long and complex.” Fifty investigators were working on it and would interview workers from five companies hired for the renovations to the cathedral’s roof, where the flames first broke out.