One thing that would help would be to change the law to require scrap metal dealers to ask sellers to show identification and receive payments into their bank accounts.
If the source of the blast was close to the door, closing it almost immediately, would it not take a finite time for the fireball to propagate the the rest of the house (round corners etc.) before it showed itself?
Another possiblity would, I suppose, be a 2-stage process - an initial explosion, however caused (maybe 'directly electrical') which closed the door and ruptured the gas incomer, rapidly followed by the consequential gas fireball.
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