J.K. Rowling makes the culprit very unclear and ambiguous. By the end of this chapter, when Hermione is conjuring up ways to slip into the Slytherin common room and prove that Malfoy as Slytherin's heir, the reader would have a good reason to suspect Malfoy; we know that he has called Hermione a Mudblood and grinned at the immobile petrified cat. He has ancient family roots in the Slytherin House, and he seems just malicious enough to be responsible for the harm caused. Furthermore, Percy Weasley is in a suspicious place at a suspicious time, and is extraordinarily upset at Ron for being there, too. Both of these boys could be seen as possible culprits. Rowling frames many different characters as possible culprits, including Harry himself, before she reveals the truth.