Hello! Are police back probing who sent this? (4,4)
I believe the answer is:
hate mail
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for hate mail that I've seen before include "Offensive correspondence" , "Abusive messages" , "Poison-pen letters" , "communication with animosity - with 19a" , "Nasty letters" .)