Young lovers Jens Söring and Elizabeth Haysom were convicted for the 1985 murders of Haysom’s parents, but The Promise highlights the trial’s inconsistencies

He was the academically inclined son of a German diplomat, she the daughter of a wealthy society couple, educated in a string of European boarding schools.

Jens Söring and Elizabeth Haysom were convicted for the brutal 1985 murders of Haysom’s parents, who were bludgeoned to death in their home in Bedford County, Virginia.

Thirty years after the former lovers were put behind bars for the killings of Derek and Nancy Haysom, a new documentary highlights the extent to which Söring’s 1990 trial was a jumble of omissions and inconsistencies – and casts doubt on the veracity of his conviction, which was based purely on his initial guilty plea and the print of a bloody sock found at the crime scene.

The case has become a diplomatic issue: the German chancellor, Angela Merkel, has expressed her wish for Söring to be repatriated and has raised her concerns about the case in one-to-one talks with Barack Obama.

It has also become a political issue in the US: in 2010 former governor Tim Kaine – who is Hillary Clinton’s running mate – granted Söring’s request to continue his prison sentence in Germany.

But the decision was reversed by Kaine’s Republican successor, Robert F McDonnell, on his first day in office. Since then Kaine’s decision has been cited by Republicans as evidence of what they say is questionable judgment.

Facebook Twitter Pinterest Jens Söring interview, 2014. Photograph: PR

Terry McAuliffe, Virginia’s current Democratic governor is considering a new request from Söring. The petition throws doubt on the German’s guilt, drawing on DNA tests – unavailable at the time of the trial – to show that blood found at the scene does not belong to Söring.

While Söring waits for a response, the new documentary makes its US premiere at the Virginia film festival on Saturday at the University of Virginia in Charlottesville – where Söring and Haysom first met in 1984. Considerable resentment has been expressed locally that it is being shown at all.

The Promise is also due to be serialised in six parts in the UK on the BBC in March.

German film-makers Marcus Vetter and Karin Steinberger spent more than four hours interviewing Söring, now 50, at Buckingham correctional center in Dillwyn, Virginia, where he is serving two life sentences.

In the film, he admits making “lots of mistakes” but insists on his innocence.

Derek and Nancy Haysom were found stabbed to death in their house on 3 April 1985. At first, police seemed not to consider either Söring nor Haysom as potential suspects. But when Söring was called in for questioning, the couple fled the country, initially heading to Thailand, then Europe.

They were eventually arrested for cheque fraud in Richmond, on the outskirts of London, in April 1986. After four days of questioning – with no lawyer present – Söring confessed to the murders.

Later he recanted his account, saying he had only confessed in order to protect Elizabeth Haysom from the electric chair, believing that he enjoyed diplomatic immunity through his father.

“I thought I was a hero,” he says in the film. “I thought I was a great guy and I was saving her life. What I saw before me was Elizabeth, a beautiful young woman who I deeply loved and who was in danger of being executed,” he said.

Facebook Twitter Pinterest Elizabeth Haysom enters a plea of guilty on two counts of being an accessory before the fact of the murders of her parents on 24 August 1987. Photograph: Dan Doughtie/AP

In a landmark case, the European court of human rights ruled that he could only be extradited for trial in the US if it would guarantee he would not end up on death row. US authorities agreed the death penalty would be ruled out as a sentencing option.

Back in the US, Elizabeth Haysom pleaded guilty to two counts of accessory to murder before the fact. Haysom, a former boarder at Wycombe Abbey in Buckinghamshire, is serving 90 years at the Fluvanna correctional center for women in Troy. She communicated with the film-makers by letter, but declined to give them an interview.

In a 2010 statement issued to the Press Association when Söring’s extradition to Germany was being considered, she said: “If Jens was innocent I would shout it from the highest mountain peaks but he isn’t innocent, he is just as guilty as I am.” In a 2015 interview with the New Yorker she said she hoped he would nevertheless be repatriated.

The new documentary does not reach a conclusion over Söring’s guilt, but it raises a string of questions about the investigation and trial – the first criminal trial in the US to take place in front of live TV cameras.

Why was Judge William Sweeney allowed to take the case, despite the fact he had been a friend of the Haysoms and had given a press interview saying he was surprised that Jens had allowed Haysom to “dare” him to kill her parents?

Why did Sweeney dismiss attempts to examine repeated claims – based on photographic evidence that subsequently disappeared – that Elizabeth had been sexually abused by her mother?

Why were no attempts made to track down the hire car Haysom and Söring had rented on the weekend of the murders, which was later found with bloodstains in it?

What happened to an FBI profile of the offender?

Why were eyewitnesses, interviewed in the film, not called to give evidence at the trial?

The bloody sock print – the main piece of evidence on which Söring was convicted – was deemed by a tyre mark expert to be Söring’s, even though it was too small for his foot.

“Despite having researched this case for 10 years,” film-maker Karin Steinberger told the Guardian, “I still don’t know what happened the night the murders were committed. I don’t know if he’s innocent or guilty. But I do know that this was not a judgment beyond reasonable doubt as another man’s blood was found at the scene.

Facebook Twitter Pinterest Jens Söring at trial in 1990. Photograph: PR

“The movie doesn’t say Jens is innocent, it says there are lots of things that remain unanswered. Not least the question: ‘is the real culprit still out there?’”

The two main investigators who worked on the case came to opposite conclusions, with one convinced of Söring’s guilt, and the other believing that the wrong man is behind bars. Gail Marshall, the former deputy attorney general of Virginia has been one of the most prominent Americans to suggest Söring had what she called a “very unfair trial”.

Söring has become something of a cause célèbre in Germany, triggered by Steinberger’s initial interview with him for the Süddeutsche Zeitung in 2006. He has a Facebook page of supporters and recently more than 120 German MPs – together with the former president Christian Wulff – signed a petition for him to be brought home. Some of them have even visited him in prison.

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For years he has sent a Christmas card to Angela Merkel. Reportedly, she also sent him one last year.

The US reluctance to allow him to return to Germany and carry on his sentence – his supporters say it would save the US taxpayer hundreds of thousands of dollars – has been widely interpreted in Germany as a rebuke of its more lenient judicial system and driven by the political calculus of appearing tough on crime.

“There are many people in the US who are out for revenge, who don’t believe in mercy, and say he should die there,” Steinberger said. “In Germany the belief is that after being punished, people should be given a second chance.”