Vaughn's first thought was DeSantis. He did not believe the North American wizards could sneak into the Ancient Castle and attack a Hogwarts professor right under his and Dumbledore's noses.
The Ancient Castle held far too many secrets and had far too many spells cast upon it. As headmaster, Dumbledore might not know everything happening inside it at every moment, but detecting an uninvited visitor was hardly difficult for him.
Vaughn did not have that level of authority, but he had his own means: the Dimensional Gate.
He had left numerous "spatial nodes" for the Dimensional Gate around Hogwarts. With the help of his Personality Armament, he used those nodes to monitor the Ancient Castle, inside and out.
Those spatial nodes were like surveillance devices, or cameras, while Vaughn's Personality Armament was more like a "double" dedicated to watching the "security monitors."
So if the attacker was an outsider, it could only have been DeSantis. Vaughn's surveillance was not yet capable of exposing even Legendary Wizards wherever they hid.
Dumbledore had also admitted that DeSantis was unlike a conventional Legendary Wizard. Even now, he could not be certain whether those beings were Legendary Wizards at all, or whether they could still be called human. As such, he could not keep track of them at all times either.
If DeSantis was the attacker, the only possible motive was to create chaos and obtain Tom's Diary.
Yet Vaughn always felt that explanation was a little strained, because DeSantis and the North American wizards had not even caught a glimpse of Voldemort yet.
Their supposed cooperation was really just an agreement with the Death Eaters and the pure-bloods led by Jacob. Until Voldemort appeared and they established contact with him, that cooperation remained nothing more than an ideal.
Besides, the Pure-blood Alliance had