The identity of 'H' - the mysterious Line of Duty villain who's at the head of a network of corrupt cops - appears to have been revealed during Sunday's episode.
Fans believe that the culprit was revealed during the mammoth 30-minute long interrogation scene, where Jo Davidson was interviewed by Ted Hastings, Steve Arnott, and AC-3's Patricia Carmichael.
During the interview, Patricia Carmichael tapped her pen in frustration, following long periods of Jo replying "No comment" to every question.
Viewers pointed out the fact that the Superintendent tapped her pen four times in a row, which is Morse code for the letter 'H'.
This has led people to arrive at the theory that Carmichael is H, aka the fourth (wo)man who is in charge of corrupt cops.
The theory might well be a strong one. Audiences will remember that, at the end of season 5, Steve Arnott reviewed the footage of Dot Cottan's dying declaration and saw that he had tapped four times using his hand, indicating there was four 'H's. This shows that the Line of Duty writers have a penchant for dropping in Morse code as hints for the audience.
It also comes as fans have compiled further evidence that Carmichael might well be H. Earlier this series, Jimmy Lakewell said cryptically to Steve Arnott to "look beyond the race claim" to find H. This led people work out that, if you take the letters 'race claim' out of Carmichael, you are left with one letter: H.
Viewers reacted to the Morse code hint on Twitter, with many convinced that it solved the key to the entire series.
One said: "PATTY C JUST TAPPED 4 TIMES MORSE CODE FOR H??"
Another added: "Don't mean to alarm anyone but at Carmichael's pen? Tapped four times and H in Morse code is • • • •"
While a third said: "Good theory about Carmichael tapping H in morse code during the interrogation."
We'll be left to keep guessing for another week however.
The final episode of Line of Duty airs on BBC One at 9pm on May 2.
