Who Killed Cassandra (Was it you?)

The story starts when a boy move in to her Sister's Dorm to investigate due to it's death.
He told no one about who he really is and his true intentions in moving to the Dorm.
Even to his new found friends who happened to be his Sister's peer.
But his new found friends are not just ordinary. Some of them knew what he's keeping.
They became a one group and continue the investigations. Yes, they investigated as well.
But when their suspects all proved(by their secret investigations) to be innocent and their investigation starts to become hard and got to the state where they Suspect Innocent People.
What will they do? Will they continue? Or stop and surrender all what they've started?
But the BIG question is?
WHO KILLED CASSANDRA.