YA murder mystery

read this book years ago so can't remember much

it was this murder mystery where this young girl is trying to find out who killed her best friend I think she left her a letter and as she starts to solve it she start to fall for the murderer (who is really into photography) and then in one of the last chapters is him trying to kill her and the MC by lighting the house on fire because he wanted them to stay together forever

I think there was also. a love triangle between her and the murders brother and he ends up saving her but i cant rember

This might be "No One Else Can Have You" by Kathleen Hale

touchstone: No One Else Can Have You