The Caller by Karin Fossum

One mild summer evening, a young couple are enjoying dinner while their daughter sleeps peacefully in her stroller under a tree. When her mother steps outside she is stunned: the child is covered in blood. Inspector Sejer is called to the hospital to meet the family. Mercifully, the child is unharmed, but the parents are deeply shaken, and Sejer spends the evening trying to understand why anyone would carry out such a sinister prank. Then, just before midnight, somebody rings his doorbell. No one is at the door, but the caller has left a small gray envelope on Sejer’s mat. From his living room window, the inspector watches a figure disappear into the darkness. Inside the envelope Sejer finds a postcard bearing a short message: Hell begins now.