James stayed with the team for several hours, unable to get a break even to owl Lily to let her know he may be working very late tonight and not to expect him.
First he went over every inch of the crime scene, looking for a strand of hair, a button from a jacket, a drop of blood, anything from the killer, anything at all that could lead to a suspect, even. Nothing. The only thing they would have would be the vampire's saliva, and that would be mixed with what little remained of the victim's blood and tainted too badly to make an identification. Besides, they had no suspects to take saliva from for a comparison.
Then he went with the bodies to the Ministry lab to observe the examination, which only confirmed what he'd already suspected. Both victims had been fed on heavily for at least a day before being taken to the graveyard and hung. The hanging might have been what finished it, but the weakening from the blood loss was what had killed them, in all reality. The bites were from a genuine vampire, and this time the examiner was able to confirm that all the bites--from these two victims, and from the Hogwarts victims--were from a single individual. So they were not dealing with a group, but one vigilante vamp.
Beyond that, all the examiner could really tell him immediately was that the victims were not related. Further investigation might reveal more.
After that, James returned to the crime scene to oversee the final clean up and to interview a few witnesses, all of whom turned out to actually be curious cats who wanted to ask more questions than answer them, and were really not useful at all. James had taken their names and would give those names to a Hitwizard later to deliver them all with tickets for something dealing with obstructing an investigation. Something. He couldn't just let people go around interfering.
Finally, he penned a quick owl to Lily to tell her work had suddenly gotten busy and not to wait up for him, then took himself to the Head's office. He knocked on the door before opening it.