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Series 6 of the intense police drama sees the anti-corruption squad following Joanne Davidson (Kelly Macdonald), whose unorthodox methods when investigating the murder of journalist Gail Vella (Andi Osho) have aroused suspicion.
But in typical Line Of Duty style, there's far more to this case than meets the eye - with DI Steve Arnott (Martin Compston) discovering a snippet of the podcast Gail was working on.
After an interview with Gail's former producer, Steve discovers that Gail was interviewing senior figures for venture, which focused on police corruption and cover-ups, before she was killed. While Gail's original laptop appears to have been stolen, a decoy laptop has a slither of an audio file of her podcast - which features Gail speaking to an unnamed voice.
The mysterious voice only says: "There are some people we can't challenge," before cutting out.
While Steve is perplexed at the voice, Line Of Duty fans reckon they've cracked who it is - with many speculating that it's Jimmy Lakewell (Patrick Baladi) speaking.
For those of you who need a refresh, Jimmy Lakewell is the lawyer from Line Of Duty season four, who defended both Roz Huntley (Thandie Newton) and her husband.
Jimmy was revealed to be one of the attackers named collectively as 'the Balaclava man' with links to the OCG (organised crime gang, everyone), that attacked Steve Arnott. He was imprisoned at the end of Season 4 after pleading guilty to perverting the course of justice.
Fans took to Twitter to discuss their detective work worthy of AC-12.
"Jimmy Lakewell was *for sure* the voice on that recording," one person wrote.
"That partial audio from Vella's laptop must be Jimmy Lakewell from season four," said another.
"After listening to the audio clip again, I think it's Jimmy Lakewell," a third concurred.
Of course, with this show being Line Of Duty, after all, we can't guarantee anything...although we can't deny the likeness.
Another theory doing the rounds on social media is that Kate Fleming (Vicky McClure) is already on to Ryan Pilkington (Gregory Piper).
The former AC-12 detective was spotted looking him up on her computer - obviously recognising him from when she interviewed him in 2012.
You can read more about that here.
Line of Duty continues Sundays at 9pm on BBC One.