It's this missing student, Kaitlyn Johnson - who was never even registered as a student at Elmbridge -, who is suspected of being the person behind the fire. Kaitlyn's story is one that has captivated people in the over two decades since that horrible day and speculation has become urban legend. But now evidence of Kaitlyn's existence has finally been found.
Carly and Kaitlyn were always together. Carly got the day and Kaitlyn got the night. Their parents knew, their sister knew, but they were told never to let anyone outside of the family find out. When their parents were killed in an accident, though, Carly/Kaitlyn found themselves institutionalized at the Claydon Mental Hospital. Deemed to be on the upswing and not a danger to herself or others, "Carly" was allowed to attend Elmbridge during the school year. But during her final year at the school things started to get undeniably out of hand.
Dawn Kurtagich's debut is quite different. The reader never really gets a full explanation as to what's going on either with Carly/Kaitlyn or the fire. This is because the book is laid out as recovered diary entries, emails, IMs, and video recordings from their final year at the school.
Carly and Kaitlyn are deemed by their doctor to be a unique case of DID - dissociative identity disorder. Their doctor believes, in spite of what Carly and Kaitlyn tell the reader through their journals, that Kaitlyn manifested after the accident that killed Carly's parents. Indeed, the obvious implication that their younger sister knows about both personalities (if our narrators' journals can be trusted that is) does support the idea that both of them have been present their whole lives.
Notice I said if our narrators can be trusted. That's kind of key and it makes The Dead House a deliciously mind bending read! The way the story is presented gives the reader much to mull over. It's clear the police believe Kaitlyn set the fire and killed those students. The story that plays out via the journals could be taken that way or not, depending on where your sympathies lie. You could, for example, follow Naida's beliefs about the situation, or you might subscribe to Dr. Lansing's theory.
Either way, the story is limited and left up to your own imaginings to a certain extent, which means that each reader can come away with something different from the experience.
Personally, I'm not one to brush the fact that the school continues to claim victims well beyond Kaitlyn's time there under the rug. Just sayin'!