If it was brought down, then most likely it was saturated with a wide area wide bandwidth high power 'everyone be damned' signal. Not by firearms of any type.
No not firearms... I wouldn't bet $5 on that..
A tech friend of mine suggested that the russian ELINT system (allegedly in iran's arsenal) could potentially jam the radio altimeter and INS/GPS trancievers on board with appropriate signals to make the drone think it is somewhere else in 3D space.