Unless your gun nut was loading ammo for something like this there's no way anyone would have enough powder on hand to produce the explosion that happened in Indy. A single pound of the powder handloaders use would load a thousand rounds of anything that anyone would be likely to use, and for their own safety no sane loader would have more than a couple of pounds anywhere in a single place.

The most likely cause probably is a gas leak. The homes were very large, and if the family was all away the one that blew was probably fairly tightly closed up. While a gas furnace might have "four safeties" that would have to fail if the furnace was properly installed and in good shape, blowing out the pilot light on a gas cooking range could accumulate a fairly large amount of gas in a day or so. Any leak in any pipe could do the same. No indication was given as to how long the family was gone, at least that I've seen.

In order for enough gas to accumulate for the blast that occured, the house would have needed to be closed tightly enough that little would have leaked outside, and adjacent houses were well separated, so the cited fact that there was no "gas smell" isn't definitive.

If a "criminal act" is involved, it's most likely someone just opened a valve, or broke a pipe, and walked off. Eventually it would blow.

Any explosive material other than natural gas capable of the damage seen in the photos would leave chemical traces that the ATF boys would have detectors to identify within the first half hour of arriving at the scene - even if it took them a week to get there.

Of course that's just speculation - (by a US Army trained ammunition officer, if that's of significance).

John