Could a terrorist have modified a semi-auto AK to go full-auto? Not a professional gunsmithing job - just enough to make it work full-auto but the rate of fire might be different from that of a rifle that was assembled with all the right parts in the factory?
Could one of those “bump fire” gimmicks have been used?
Yes, though semi-autos converted in this manner usually have all sorts of problems and don’t do sustained fire well. In fact, they usually break.
Too high a sustained rate of fire for a bump fire device, IMHO.