Here is all the evidence so far...
Where does Kate know Ryan from?
Back in Season 1 when Ryan was a mere teenager he was brought in for questioning after Steve Arnott was kidnapped by gang leader Tommy Hunter. Ryan found himself caught up in the drama and even tried to cut off Steve's fingers while he was tied up.
In episode two of Season 6 Ryan could clearly be seen keeping his face hidden while he was in the same room as Steve. While Steve and his AC-12 team raided Hillside Lane Station, Ryan kept his back to Steve, knowing full well that he won't have forgotten the face of the teenager who tried to cut off his fingers.
How are Kate and Ryan working together now?
It was revealed at the end of Season 5 that Ryan had taken exams and had an interview to become a police officer. But while he appeared to his new bosses as an upstanding member of society, little did they realise he was actually a well-respected member of the OCG and was going into the police as their mole.
When PS Farida Jatri requested a transfer in episode one of Season 6 following her romantic split from DCI Davidson, Ryan was the newly qualified police officer who filled her role.
The first time Kate came face to face with Ryan as a police officer was at the Carl Banks murder scene in episode two. Kate had a flash of recognition at the moment they met, while Ryan had a look of pure hatred on his face as Kate turned her back.
Later the pair met again in the office and Kate grilled Ryan about whether they had worked together before. Ryan fobbed Kate off by asking her whether she gave a talk at his police college, managing to cleverly avoid the question.
What clue have Line of Duty fans spotted that hints Kate has worked out Ryan's true identity?
Following Kate's run in with Ryan at the office, there was a one second moment where Kate could clearly be seen searching Ryan's police profile on her computer screen.
It was a blink-and-you'll-miss-it moment, but some eagle eyed fans managed to see it nonetheless.
Does this mean that Kate has already worked out where she knows Ryan from? Or was she trying to simply see if they had worked together before?
If Kate has worked out that he is part of the OCG, how long will she hold on to this new-found information? Will she keep it under her hat until it proves useful? And could this mean Kate is actually somehow working undercover for AC-12 after all? It seems there is a lot more to come from this latest twist.
Line of Duty continues on BBC One on Sunday at 9pm. You can catch up on BBC iPlayer.
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