Documentary-makers claim to have identified two prime suspects in the 1974 IRA Birmingham pub bombings.
The explosions at the Tavern in the Town and Mulberry Bush killed 21 people and injured another 220.
One of the men named in the ITV Exposure programme, to be broadcast on Monday night, is convicted IRA bomber James Francis Gavin, who has since died.
Gavin, a former British soldier, is alleged in the documentary, Hunt For The Birmingham Bombers, to have been one of the people who planted the bombs.
The second suspect is shown being asked about his alleged role by programme-makers outside a Belfast supermarket.
He declined to comment and his lawyers told the programme: "Our client denies all the allegations ... and does not intend to respond any further to the unfounded allegations you have made."
The documentary will suggest that the Crown Prosecution Service previously examined the case against the new suspect and concluded there was insufficient evidence to charge him.
A botched criminal investigation by West Midlands Police immediately after the atrocity led to the jailing of the Birmingham Six - one of the most infamous miscarriages of justice in English legal history.
Their convictions were quashed in 1991 after a long battle by campaigners, including MP and journalist Chris Mullin.
Nobody else has been brought to justice for the attacks.
Julie Hambleton, whose 18-year-old sister Maxine was killed in the blasts, has for years led the Justice4the21 campaign and was interviewed for the programme.
Ms Hambleton said: "What do I want? Me, personally, I want the bastards who killed my sister and the other 20 to be brought to justice, short and simple."
She added: "We could have walked past him when we were in Belfast.
"When people ask you how'd you feel if you met them or saw them, you can never answer that question."
The Court of Appeal ruled last week that a coroner had been right to exclude identifying alleged perpetrators of the attacks in fresh inquests.
The documentary will be broadcast at 10.40pm on Monday.