I need advise. My niece who is 3rd grade was in computer class and they were doing paperwork and…

I need advise. My niece who is 3rd grade was in computer class and they were doing paperwork and left her seat and when she came back to it she found the words "im going to kill you" typed on her computer. She told the teacher and she asked but none of the kids raised their hand. So she told my niece she was going to try to find out who did it. The teacher didn't call her mom or anything. Should she have called or not? What would you guys do?