i’ve got one that i’ve desperately been searching for for YEARS! no rush to reply but i thought i’d add myself to your long list of mystery books to find. i read it at least 10 years ago, i as far as i remember it was a fairly short book about a kid experiencing a school shooting/attack and frantically trying to find their twin/sibling, with the twist being that they had no sibling and (if i remember correctly) they were the one that had done the shooting.
Lost Classics of Teen Lit: Name That Book! 