The killer is...
I believe the killer is the maintenance worker at the police station
You'll notice when they're questioning the second suspect a man enters the room carrying a tool box, wears gloves (smooth hands). Nobody seems to take much notice of him which is fairly odd as they're in the middle of interrogating/torturing the suspect. As he fixes the boiler of whatever it is, he glances over his shoulder a couple of times listening to the confession but were never given a good look at his face, he gets up and leaves walking back past the 2 detectives and up the stairs say the third detective waiting outside, not once does anybody look or even acknowledge his presence, my guess is his plain appearance didn't call for much attention. Also working in a police station he'd might have some knowledge of how to commit and get away with the murders
Anyways just imo, as soon as I saw that I thought has to be him