Because so much of the Requiem as normally performed was written or completed by someone other than Mozart, scholars have spent more than two centuries trying to separate his hand from Sussmayr's, work made harder by the loss of most of Mozart's working sketches for the later movements. What survives in Mozart's own hand with reasonable confidence is the Introit fully orchestrated, and vocal parts with figured bass for the Kyrie and most of the Sequence and Offertory; the Sanctus, Benedictus and Agnus Dei exist with no confirmed surviving Mozart material at all, raising the question of how much of their musical material, as opposed to just their orchestration, is actually Sussmayr's own invention.
Sussmayr himself later claimed in a letter that some of the material for these later sections came from verbal instructions or sketches Mozart had given him before dying, sketches that, if they ever existed in written form, have not survived to be checked. Modern scholars remain split on how much weight to give that claim, and on whether Sussmayr, a competent but by most accounts less gifted composer than Mozart, was capable of matching Mozart's style as closely as some passages suggest, or whether those passages preserve genuine (if now lost) Mozart material passed to him by other means.
Several later composers and musicologists, dissatisfied with aspects of Sussmayr's completion, have produced their own alternative completions since the late twentieth century, none of which has displaced Sussmayr's version as the standard concert text, a testament less to its unquestioned authenticity than to how thoroughly it has become the Requiem audiences expect to hear.