When I heard about gunshots being fired immediately before the explosion, I thought of tannerite. A large enough amount of tannerite in an enclosed space could make one heck of an explosion --- it doesn't take all that much. But tannerite does not create a fireball. Aside from tannerite, which is gotten fairly easily (Academy Sporting Goods, and many other places), I have to think that explosives could not be bought in large enough quantities to do damage like that, without attracting someone's attention. Maybe whomever was pulling Warner's strings (assuming he had enablers) got him to use tannerite, then, ahem, "did their own thing" at the same time.
I've never used tannerite but my son and I enjoy watching YouTube videos of guys "blowing [stuff] up" --- they lay waste to things like refrigerators, microwave ovens, and old junked vehicles. Just good clean redneck fun.