Writers Harry and Jack Williams revealed the culprit in episode five to play “a different game narratively”

Harry Williams: Traditionally, ep 8 would be the one where you go okay, ‘here’s the suspect, we’re finally going to hone in on who it is’, so for us we were like, well if in this season we can do that slightly early, we can play a different game narratively and that can be the surprising thing and there’s a whole other story to tell there with the antagonist.

Jack Williams: We always try to think about expectation and what people watching the show might expect to see. In series one people are very much speculating on who did it and who will it be and taking bets on who it will be that it just felt interesting to us to go ‘what if that’s not what it’s all about. What if it’s a bit bigger than that?’ Who it is isn’t always a question that’s particularly thrilling to answer because it’s like pointing a finger and going ‘that guy’, but why they did it or who they are, and the fact we now have three more hours to explore that side of it.

You weren’t supposed to guess who the culprit was before episode 5

Jack Williams: I don’t think there’s any way you could know that at all. And that’s a good thing. Because we do it so early, there’s time in the next three episodes to really explore how and why he’s connected to everything else that is going on. I mean, if you play the game enough and you go ‘I vaguely recognise him and he hasn’t had that much to do… which I did see some people going ‘he’s quite a good actor, he hasn’t done very much’ but you can’t avoid that. You have to cast someone really good. I think we tried to use him as much as the story required. There are army press liaison officers, we met one when we did the research for this show and thought that’s quite a weird, specific job and he’s got quite a good reason to sit with the family, he’s organically involved at least, but no, I don’t think you could really guess.