But Galerius, not satisfied with the tenor of the edict, sought in another way to gain on the emperor. That he might urge him to excess of cruelty in persecution, he employed private emissaries to set the palace on fire; and some part of it having been burnt, the blame was laid on the Christians as public enemies; and the very appellation of Christian grew odious on account of that fire. It was said that the Christians, in concert with the eunuchs, had plotted to destroy the princes; and that both of the princes had nearly been burnt alive in their own palace.
Diocletian, shrewd and intelligent as he always chose to appear, suspected nothing of the contrivance, but, inflamed with anger, immediately commanded that all his own domestics should be tortured to force a confession of the plot. He sat on his tribunal, and saw innocent men tormented by fire to make discovery. All magistrates, and all who had superintendency in the imperial palace, obtained special commissions to administer the torture; and they strove with each other who should be first in bringing to light the conspiracy.
No circumstances, however, of the fact were detected anywhere; for no one applied the torture to any domestics of Galerius. He himself was ever with Diocletian, constantly urging him, and never allowing the passions of the inconsiderate old man to cool. Then, after an interval of fifteen days, he attempted a second fire; but that was perceived quickly, and extinguished. Still, however, its author remained unknown. On that very day, Galerius, who in the middle of winter had prepared for his departure, suddenly hurried out of the city, protesting that he fled to escape being burnt alive.
Source: Of the Manner In Which the Persecutors Died (New Advent)