Does Line of Duty series 6 trailer hint at identity of fourth corrupt officer?

But arguably the most eagerly awaited aspect of the new series is the possibility of finally finding out the identity of the fourth corrupt senior officer, and some fans think the recently released trailer might have delivered a big clue.

In the final moments of the trailer, new character Chloe (Shalom Brune-Franklin) is seen asking Steve Arnott (Martin Compston) “Do you still think there’s a fourth man out there?” to which he replies, “Man, or woman.”

And crucially, the trailer then cuts to one final shot of Kate Fleming (Vicky McClure) – could this suggest that the fourth corrupt officer is none other than Kate herself?

It would certainly be a major twist for the hit drama, but it's not the first time during the promotion of the new series that the show has hinted the usually dependable Kate might not be quite what she seems.

When a teaser trailer arrived at the end of last month, fans were quick to point out that it seemed as if Kate was not on the same side as Ted and Steve.

A brief clip showed her standing alongside new character Joanne Davidson (Macdonald) and appearing to reach for her gun after Ted declares, "This is Superintendent Hastings, you'll be treated fairly with the full protection of the law."

It's possible that Kate is simply back undercover, though previous it'd been implied that her days as an uncover officer might be over. With Steve and Ted have both previously found themselves on the receiving end of some tricky questions before could it now be Kate's turn to be interrogated her own AC-12 colleagues?

And there would certainly be a lot of questions for the show to answer if Kate is indeed "H" – chief among them regarding why she went after corrupt cop Matthew 'Dot' Cottan in series three.

There are still several other possibilities regarding the identity of the last remaining H as well – including Hastings himself, though he appeared to be exonerated at the end of season five after having emerged as a suspect, series creator Jed Mercurio has refused to rule out the possibility that Ted might still be a "bent copper".

Another prime suspect is Rohan Sindwhani (Ace Bhatti), who was introduced as Central Police's Police and Crime Commissioner in series five.

In the new trailer, Rohan is seen telling the press that, "There is no corruption in this police force" – a claim we know to be just a little misleading, and indeed the very next thing we hear is Ted fuming, "A bare-faced liar promoted to the highest office!"

And of course, there's also Joanne Davidson, the new character played by Kelly Macdonald who is the subject of the latest AC-12 investigation and whom Mercurio has described as "AC-12’s most enigmatic adversary yet."

Joanne is questioned as a result of her unconventional conduct while leading a murder investigation, and it seems certain that she will face some strong accusations during the series – but the possibility exists that she might be an even bigger player in the police corruption scandal than we (and AC-12) first suspect.

One thing's for sure, the next series is bound to be full of as many twists and turns as usual – and we'll doubtless end up with a whole heap of brand new theories once the show makes its return later this month.

Line of Duty returns on Sunday, 21st March at 9pm on BBC One. Take a look at the rest of our Drama coverage, or check out what else is on with our TV Guide.