The story is: he entered the bathroom and soon after it exploded. Whatever he used, he had with him when he got there.
And that's the key, 'the story is'. I wonder, would a floor plan of the Horton's and the time of detonation be important to sorting this out? It has happened here in the deep South that a 'fry station' under an industrial hood was used to cook up Meth during 'store closed' hours. Some explosives have poisonous gases as by products during 'cooking'.