Offer both of them full immunity to fess up, if they know, in the interest of making sure the house is safe versus there being an unknown fire risk somewhere/somehow. No punishment or repercussions other than they know not to do whatever it was again, where all bets will be off.
They are already off scott-free now, and you don't know what happened. Might as well let them off the hook and have the peace of mind of knowing what happened.
ETA: throw in the bluff that if you don't figure out what happened, you'll probably have to call tie fire department to do a check and a trained professional might figure it out and have it come back on one of them, which will mean trouble.