We have a separate thread for this one:
Investigation of the cause is in the early stage but what I read:
Witnesses described a scene of absolute chaos. Two survivors told French broadcaster BFMTV that the fire appeared to start when a staff member, carried on another’s shoulders, held a lit sparkler or candle in a champagne bottle too close to the low, wooden ceiling.
“In a few dozen seconds, the whole ceiling was on fire,” one witness said. “Everything was made of wood.”
As the flames spread, a flashover—a phenomenon where combustible gases ignite simultaneously—triggered at least one explosion. The rapid escalation led to a desperate scramble for the exits.
The Basement Trap: Many victims were reportedly in the basement nightclub and faced a narrow staircase and a single exit as the ceiling above them began to collapse.
Latest estimate is 40 dead and 100 injured, but with lung injuries from inhaling toxic gases I think it likely more will die over the next several weeks.