Did police spy on Welsh miners during strike?
Did undercover police officers infiltrate the National Union of Mineworkers during the 1980s strike?
Former miners hope that question will be answered by the current public inquiry into covert policing in the UK.
The suspicion that specially-trained police spies were active in Wales during the 1984-85 strike has hung over mining communities for years.
One former senior NUM member says he knows of at least one person who left Wales suddenly once the strike ended.
In 1968 the Metropolitan Police set up an undercover unit called the Special Demonstration Squad (SDS). Part of Special Branch, its purpose was to infiltrate protest groups.
After it emerged in 2013 that SDS officers had spied on the family of murdered teenager Stephen Lawrence, Theresa May - then Home Secretary - set up the Undercover Policing Inquiry (UCPI) into how covert policing operations were run.
The NUM has been given core participant status at the inquiry, along with organisations like London Greenpeace, Reclaim the Streets and Cardiff Anarchist Network. Core participants can give evidence and hope to find out if, and to what extent, they were spied on.
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One former high-ranking miners' representative told BBC Wales Investigates he believes a handful of specially-trained undercover officers were active in Wales during the miners' strike.
Tyrone O'Sullivan, former NUM branch secretary, says he and other miners are convinced that undercover officers infiltrated their ranks during the strike - and were feeding information to the police, and ultimately back to the government.
"We were a huge threat (to the government). They spent billions to defeat us," said Mr O'Sullivan. "They tapped our phones, they infiltrated into us. That didn't happen because of the strike, it started years before.
"You can't do it (infiltrate the community) a week before the strike - you can do it two years before the strike. You can be part of the community that way."
Mr O'Sullivan, now chairman of Goitre Anthracite Ltd, owners of Tower Colliery near Hirwuan, says he has strong suspicions about one individual, who he is not naming.
"Definitely now, at the time no, but with what happened after the strike we thought there was a reason for this. He'd gone away too soon, he'd left too soon. He'd left a girl he had made so many promises to.
"I think it was very well organised - it wasn't the PC up the street. This was a far larger organisation. These people were trained purposely not only to undermine miners but to infiltrate everything."
Former Labour minister Kim Howells, who was an official for the south Wales miners during the strike, said there were "conspiracy theories around" during the time, including that people's phones were being tapped.
"We just assumed that they were listening or else that there were people within the ranks of miners or miners' families who the police were getting information from," he told BBC Radio Wales' Good Morning Wales programme.
The Metropolitan Police declined to comment on the NUM allegations, as they are part of the UCPI. The public inquiry, which began in 2015, is not expected to publish its findings until at least 2023.
You can watch BBC Wales Investigates Undercover Cops: Abuse of Duty on iPlayer