they went right thru the building, the engines are attached to the airplane via the wings, if anything, the wings and engines would go a few feet deeper than the fuselgae, the wings are what bare the weight of the entire airplane and at full throtle attached to the wings, they are still propelling themsleves forwar, at the speeds the aircraft is flying the entire thing can succesfully penetrate the building with the minimum aount of visual damage to the outer wall of the building, after it penetrates that first wall, that's when it starts shredding into pieces, the fire burned until the next day, the plane was loaded for an over 2000 nautical mile flight, there was enough fuel to create a fire with temperatures high enough to use the steel as a fuel, not just it be melted but vaporised. You remember the world trade center, TWR #1 was hit with this same type of aircraft, the wings engines fuselage and everything penetrated the building. An airplane is made entirely of steel, mostly aluminium and composite materials, which only take 600 degrees celcius for them to melt and burn away. With the steel of the building and the walls and office supplies, there was enough fuel to keep the fire going and for it to get stronger and stronger. The airplane litteraly disentegrated into the building and everything was burned. If you leave something on fire long enough, it will all burn away