The killer is (in the movie, not in real life case)

the little boy from the beginning. that is what the director tries to tell us

1. No pubic hair
2. small enough to pull the first victim into the sewer-kind-of-thing
3. soft hands, like a girl... you get it?
4. girl at the end says that she saw another man there. the kid from the beginning would be a man in the girls eyes after 14 or more years.
5. we can neglect the scenes showin a grown up man killing the victims. because of rashomon and/or false memories or false remembering of the situation due to stress
6. the kid is the first one we see in the movie. stylistic device
7. when they show the two-face-stupid-boy a picture of the prime suspect, the one detective puts a finger on the lower side of the suspects faces foto. the stupid boy sees a resemblance in the murderer and the man from the photo. the murderer/little kid from the beginning is probably the brother of the prime suspect. living in the same apartement with his brother, knowing when the song will be played or even himself wishing the song to play and writing the radiostation his wish... and the prime suspects knows about his littler murderer brother and tries to protect him