Harry's face took on a sympathetic expression as he jotted down Lisa's name. He'd have a chat with her next. That year. For so long as they all lived, they'd remember that year, the culmination of years, decades of terror at the hands of Voldemort. The things that went on - at Hogwarts, at the Ministry, across the country - they were terrible and he didn't want to relive them.
Perhaps he might have to, to solve this case. His mind was working quickly though. The victim likely knew Thorntwaite's habits. It would have been easy, then, to get Thorntwaite's wand, and presumably a hair to disguise himself with Polyjuice, all secure in the knowledge Thorntwaite would be asleep for hours. Perhaps he even helped that along.
"Has he been in today? Thorntwaite?" It would help even more to talk to him directly.