It probably is a gas leak or an exploding propane tank. Many of the French use propane for cooking and heating in their apartments in the older areas. The Spanish use it too, and it seems to me that every year that I have been in Madrid, there has been at least one of these explosions that result in building collapses and deaths.
That said, this must have been a rather large explosion to have caused so many deaths.
Thank you for those details, which help in understanding this grim event.