Of course, you can't see everything from the pictures, but it certainly looks like a fuel air explosion. Any type of man made explosive device would have left some type of crater as those explosions have equal energy in all directions. FAE's just seek equal pressure. If the house had natural gas in it, that's what it was I'd bet. Occam's Razor.
My guess would be that reports of gas problems/repairs might suggest a failing gas valve on a furnace or water heater, or, possibly a faulty regulator outside the house letting to much pressure in the house.
If no one was home and the basement and/or entire house filled, any speculation of ignition source? I'm thinking about my house and not coming up with anything. I could ignite it remotely with a cell phone or with a timer easily enough, which might well be why the investigation of the relatively obvious is taking so long...
There are several items in the home that could be a source of ignition without human interaction. Thermostats, pilot lights, etc.
Oven or stove pilot light, timer for lights (the owner was out of town), almost any relay not sealed or set up for explosive atmospheres, like the fridge compressor kicking in. If the mixture was just right, anything which might produce a spark.
Furnace would be my guess. Saturday was beautiful (high 60s to low 70s). It cooled at night. I speculate that the furnace may not have run all day and then kicked on at 11:10 PM. Just guessing though.
FReepers are smart.