Who killed Sutton in the Lying Games?

Well, this is a kinda tricky question since there is only ONE book of the Lying Games that have came out already... Possible clues or suspects in the Lying Games include, Laurel, Sutton's adopted sister, Charlotte, Lili, Nisha, Madeline,Gabby, Gabriella (All of these are Sutton's "close" friends. I also suspect Ethan and Garret. Ethan because he has just this innocent essence and I think that's suspicious. Garret because he is very close to Sutton as in he's her boyfriend so I just suspect him. Basically, I suspect everyone in Tuscan. The mystery will be explained more throughout the series.

People it is definitely not (according to book 2, Never Have I Ever) : Charlotte, Madeline Nisha, Laurel, and the Twitter Twins ( Lili and Gabby)