How can i found a person who sending threat mail?

hello frnd..i m in big trouble..Some one is sending many threaten mails to my boy frnd. Sender want that my b.f left me bec he lves me..if my b.f don't do this then he can Create many problems for him i.e. murder, inform the family persons, harm to family..How can i found this person.? He knows many things about ourself..How can i tackle this situation.