well the fact Jan's DNA was found on the girl's clothes, makes it a bit limited to who was the murderer. I'm sure Jan himself hasn't done it, regardless f how weird he is and what has he done - the way he's denying it, sounds extremely believable. Yes, he's got pent up frustration, but sorry M was a goady f*er towards him, I found that scene where she taunted him in his cell disturbing. She doesn't know for a FACT that he's the killer so taunting him like that was sadistic as she was in position of power. Maybe M is not such a good person after all! She's supposed to be professional and not taking her daughter's death out on everyone. At least until she absolutely knows that someone is a killer. She goaded Cullen too much also - got a fake confession as a result, just wtf!
So back to the DNA - it looks like it's Matthew then who planted the DNA. I get the impression he wants to get rid of Jan as he's in love with Henry obviously! Plus he is VERY coll and calculated compared to anyone else whereas Jan is weak and suffering from hysteria as we've seen - he was always quite a doormat. Interesting the way they portarued him in negative light but actually it's Matthew who is cold and nasty to him. Also remember Matthew was in the car at night in the park - when one of the victims was murdered and we never what he was doing there really. I thought he was hooking up with someone, but who knows now!
The only other character who could plant Jan's DNA was Henry as at least they sometimes socialise together, but I'm sure it's not Henry who killed those people (the only one he could ve killed was Grace theoretically but now looks extremely unlikely). Henry is hard to read but he's not the strongest character and seems too gentle physically speaking. whereas Matthew comes across as strong and cool-headed. Though why would he become a killer, absolutely no clue so far.
I wonder who took the photos of Jason and the contract guy - the lodger? and it was stupid to meet up in such an exposed open space! Agree that Jason looked ridiculous in that tight red poloneck! And why the heck is he telling anyone to stay away from M after having an affair, the prick!