Ex-people, plastic and so on. With a small room’s worth of air it wouldn’t have burned long, though.
More significant is just how hot it would get as it collapses. When you suddenly compress an an ideal gas (which air is a lot like) it gets hotter in proportion to it’s previous absolute temperature. Room temperature is already 273K, and the pressure down there is hundreds of time larger than at the surface. At some point the law would break down on the way, but you get the basic idea. It was probably as hot as the sun without any help from combustion