Book VI, Ch. 2 — He perished under Divine Wrath. Visions of the Emperor's Death seen by Various Individuals. Reply of the Carpenter's Son; Julian tossed his Blood aloft to Christ. Calamities which Julian entailed upon the Romans.

Ecclesiastical History · Salminius Hermias Sozomen

In the document above quoted, Libanius clearly states that the emperor fell by the hand of a Christian; and this, probably, was the truth.