Line of Duty fans think they have uncovered H after spotting a character use 'Morse code'.
The penultimate episode of the police procedural show, created by Jed Mercurio, saw Acting Detective Superintendent Joanne Davidson, boss of the suspected department, unmasked by AC-12 and secrets about her family background revealed.
Meanwhile, Ryan Pilkington was shot dead and Marcus Thurwell was discovered lifeless by Spanish police.
But during an interview with DSI Davidson, fans thought they spotted something odd.
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The AC-12 team uncovered her relation to Marcus Thurwell before DCS Patricia Carmichael revealed that Hunter was in fact her dad.
However, when DSI Ted Hastings attempted to steer the interview towards uncovering 'the fourth man', DCS Carmichael interrupted him.
Fans also spotted that DCS Carmichael looked rather stressed when DSI Davidson made any comments about the Organised Crime Group.
In fact, some picked up on DCS Carmichael tapping her pen four times during uncomfortable moments of the interview.
Ste tweeted: "During the interview, Carmichael clicks her pen 4 times... 4 clicks is Morse Code for ‘H’."
Henners wrote: "Okay so this is crazy but my aunt has just messaged me this #Lineofduty theory: Anna Maxwell was repeatedly tapping her pen 4 times in the interview, which is apparently H in Morse code?!"
And Alex said: "Anyone notice #carmichael 4 taps on the table with her pen? 4 taps, H."
The BBC One show is coming to an end after six series starring Vicky McClure, Adrian Dunbar and Martin Compston, with Trainspotting star Kelly Macdonald joining the cast for this series.
Line Of Duty returned to screens in March with 9.6 million viewers, up on last series' finale which earned 9.07 million.
The final episode on Sunday will see the search for "H" - the corrupt officer responsible for the mass conspiracy - come to an end.
