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After last night's episode of Line of Duty, fans are convinced that DCI Patricia Carmichael (Anna Maxwell Martin) is the elusive H.
During a scene in which DCI Carmichael was interrogating Jo Davidson (Kelly Macdonald) viewers noticed that she tapped her pen down on the desk four times.
Four dots represents the letter H in morse code - and it did not go unnoticed by fans.
You can watch a clip of the scene below:
Viewers are especially convinced considering the clue Dot Cottan gave before he died.
When asked to name the corrupt officer in the OCG, he indicated the letter H by blinking when Kate read through the alphabet. It was also revealed that he had moved his hands four times - indicating H in morse code.
One fan took to Twitter writing: "H in morse code is Dot dot dot dot. Carmichael tapped her pen 4 times on the desk when interviewing J."
While another said: "Carmichael tapping her pen 4 times Dot Cottan tapping his finger 4 times when he was dying 4 taps in Morse Code is.... H."
And one added: "Did you see Carmichael tapping out the morse code on the table? H."
"Carmichael tapped her pen 4 times (as we all know that's morse code for H) when Jo was talking and she reverted back to no comments after this," tweeted a fourth.
Meanwhile another added: "Morse code last night... she tapped pen 4 times in interrogation which is H and if you follow what Jimmy said, if you look beyond the race claim you will find H, race claim is an anagram of Carmichael! The letter that's missing is H."
The clue adds to the already mounting evidence that Carmichael could be H. Last week, we told you how a comment made by Jimmy Lakewell had resulted in viewers flocking to Twitter to discuss their theory.
When Lakewell said AC-12 "need to look beyond the race claim to find H" some skilled armchair detectives realised that if you take the letters in 'race claim' off the name Carmichael, you're simply left with the letter H.
One tweeted: "Jimmy Lakewell told Steve to look more into the race claim. Take away Race claim from Carmichael and what do you have left.... C A R M I C H A E L R A C E C L A I M H - I've cracked it #lineofduty."
Line Of Duty continues on Sundays at 9pm, on BBC One.
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