The Cruelty

Edward is standing near the cemetery in Sendai near the graves of the students in his crew. 

Edward: If I find out who did this, he'll pay with his own life, I promise I'll take revenge. 

Yana calls Leo. 

Yana: My teammates died !!

Leo: How !! 

Yana: Stabbed 6 times, everyone was stabbed 6 times, even Henry died stabbed 6 times !!

Leo: It can't be, thank you for letting me know! 

Yana: You told me to let you know in case something happened and I let you know. 

Leo: Thanks !! 

Leo hangs up.

Leo quickly ran to Roy because they didn't live far away, they lived in the same hotel only Roy was downstairs. 

Leo: Henry along with the other members of his team besides Yana Okabe died. 

Roy: It's impossible and Mirai's team other than Mirai is dead. 

Leo: How did they die? 

Roy: Stab 6 times. 

Leo: It was the same for Henry's team, stab 6 times.

Roy: There's something strange here. 

Leo: We have to go to Edward !! 

Leo and Roy get in Roy's car and drive to Edward. 

Roy: Edward !!!

Edward: What is Detective Roy? Wait a minute, I've seen you somewhere before. 

Roy: He is Leo is a long story. 

Edward: I see.

Roy: Is your team dead? 

Edward: How do you know this ? 

Roy: Have they been stabbed 6 times? 

Edward: I understand you're a detective, but how the hell do you know? 

Roy: Henry and his team and Mirai's team were all stabbed 6 times. 

Edward angrily says: What happens here ? 

Roy: I don't know ! 

Leo: But we'll find out, would you like to help us? 

Edward: Okay, but I want to know where your ghost powers come from. 

Leo: Okay.

Roy: Guys, I think I know why they were stabbed 6 times. 

What happened and who killed them ?