Line of Duty actor Nigel Boyle dropped a major hint that Ian Buckells was H four years ago.
The actor, whose blundering character turned out to be the "Fourth Man" during the series finale, previously suggested there was "more to him than meets the eye".
Millions tuned in at 9pm on Sunday night for the final episode of season six as the BBC cop drama reached its dramatic conclusion, bringing an end to AC-12's hunt for bent coppers.
Viewers were left utterly stunned - and more than a little disappointed - after the cop turned out to be the shadowy leader of the OCG, whose corruption had been confused as incompetence.
But Nigel had teased that they should keep a close eye on his character during an interview in 2017, The Mirror reports.
He told Digital Spy as he was promoting series four: "There's definitely more to Buckells than meets the eye!
"He's very competent, very good at what he does, he's the go-to guy, but he's certainly not a popular guy – he's petulant, and a bit of a jobsworth I think.
"I don't think people give him enough credit, just because of his personality!"
The finale saw Ian Buckles brought in for questioning after the AC-12 team found out he was the one behind the laptop with his misspelling of "definately".
After repeating "no comment" to Ted Hastings' questions, he finally responded after he was branded a "blundering fool".
"Yeah, right. I'm a blundering fool," he said with a smirk. "I'm the only one that's made mugs out of you lot."
And fans were largely unimpressed and took to Twitter to comment.
One tweeted: "We waited 10 years for Ian Buckells to be revealed as the OCG mastermind #LineofDutyFinale #LineOfDuty."
While another commented: "Waited a week for that - H being Ian Buckells! #LineOfDuty #LineofDutyFinale."
A third wrote: "NO WAY IS BUCKELLS THE TOP LYNCHPIN OF ALL THIS #LineOfDuty."
*Line of Duty is available to watch BBC iPlayer
