Narrative writing first establishes setting (place and time) and point of view, first or third person. Your reader needs to know where the crime has taken place, when the crime took place, and who is there now investigating it. As you say, it should all be done in 2 - 3 sentences. Pick a unique location for your crime scene and describe the place spatially, by physical area, showing the blood last:
As the two detectives pried open the meat locker door, the hanging shanks of beef guarded a secret: a severed human head.
Then, jump into the action by having your characters speak first (describe them later). You will want to start "in medias res," in the middle of the action. Start with a couple of detectives who are foils (or opposites): one old and cynical, one young and naive. You know the types, like Morgan Freeman and Brad Pitt in Se7en. Something like this:
The white breath of the younger detective quickened in the cold as he saw the blood congeal on the floor. This was his first assignment with his new detective badge, and his partner, who would be retiring at the end of the month, only laughed it off. "Guess who's not coming to dinner," said the veteran.
And then keep going by building suspense. Maybe have one of the detectives recognize the head. Keep the dialogue short, simple, and funny. Like a text message--except, you know, spelled correctly.