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I’ll go with gas explosion. I’ve seen first hand the results of two gas explosions , this is what you get, a leveled building. One was a residence, the other a store with a residence attached to the back. Both of them wiped slick down to curb level.

A commenter on the Daily Mail suggested a possibility of someone illegally altering the gas supply. There was a explosion in New York City a few years back. Same result, collapsed building. The cause? Building occupants tampered with the gas line in order to steal gas.

Five indicted in deadly 2015 NYC gas explosion that leveled buildings

Their trial is ingoing.

Maria Hrynenko due back in court on March 23

[Lonesome in Massachussets]

< speculation>I have no expertise or experience, but I would not be surprised if the initial explosion consumed all the oxygen, and ruptured what had been a leaking gas line. Once the oxygen level normalized, some source of ignition found its way to the leak.