Harry Dunn's alleged killer was never entitled to diplomatic immunity, the High Court has heard.
Mr Dunn, 19, was killed when his motorbike crashed into a car being driven on the wrong side of the road by American Anne Sacoolas outside RAF Croughton in Northamptonshire on August 27 last year.
Sacoolas, whose husband Jonathan Sacoolas worked as a technical assistant at the base, left the country a few weeks later after the US said she was entitled to diplomatic immunity.
The 43-year-old was ultimately charged with causing death by dangerous driving last December, but an extradition request was rejected by the US State Department in January – a decision it later described as "final".
Mr Dunn's parents, Charlotte Charles and Tim Dunn, claim the Foreign, Commonwealth and Development Office (FCDO) wrongly decided Sacoolas had diplomatic immunity and unlawfully obstructed Northamptonshire Police's investigation into their son's death by keeping the force "in the dark".
Ms Charles and Mr Dunn initially also took legal action against Northamptonshire Police but that claim was dropped in July, with the family's spokesman saying the force had been "absolved of any blame".
Dunn family fight for justice
Dunn family fight for justice
Undated family handout file photo of Harry Dunn. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
Andrea Leadsom MP and Harry Dunn family spokesman Radd Seiger arrive to speak to the media in Brackley, Northamptonshire, where the MP called on Donald Trump to personally intervene to ensure suspect Anne Sacoolas faces trial over teenager Harry Dunn's death a year ago.
File photo dated 09/10/19 of Charlotte Charles, the mother of Harry Dunn, speaking to the media after leaving the Foreign and Commonwealth Office in London, where she met Foreign Secretary Dominic Raab. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
Harry Dunn family spokesman Radd Seiger (left), Harry's mother Charlotte Charles (centre) and her partner Bruce Charles, speak with Andrea Leadsom MP (right) in Brackley, Northamptonshire, where the MP called on Donald Trump to personally intervene to ensure suspect Anne Sacoolas faces trial over teenager Harry Dunn's death a year ago.
File photo dated 20/12/19 of Harry Dunn's mother Charlotte Charles (centre) and stepfather Bruce Charles (left) outside the Ministry Of Justice in London after meeting with the Director of Public Prosecutions. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
File photo dated 15/10/19 of the entrance to RAF Croughton, in Northamptonshire, near where Harry Dunn, 19, died when his motorbike was involved in a head-on collision in August last year. One year on from his death, the family of Harry Dunn continue to fight for justice in a case which has sparked diplomatic tensions between Washington and London.
File photo dated 23/04/20 of US President Donald Trump, who was told to "reconsider its position" on the immunity given to 42-year-old suspect Anne Sacoolas, following a road crash outside a US military base in Northamptonshire which resulted in teenage motorcyclist Harry Dunn's death. One year on from his death, the family of Harry Dunn continue to fight for justice in a case which has sparked diplomatic tensions between Washington and London.
Tim Dunn (far right) and his wife Tracey Dunn (second right), as they gather with friends and family at Portland Bill to scatter his son's ashes in his favourite place in Weymouth, Dorset, a month before the anniversary of his death.
File photo dated 20/12/19 of the family of Harry Dunn (left to right) mother Charlotte Charles, stepfather Bruce Charles, family spokesman Radd Seiger and father Tim Dunn. Harry Dunn's parents have dropped their legal claim against Northamptonshire Police after concluding the force are "absolved of any blame" following their son's death.
Harry Dunn's mother Charlotte Charles arrives at the Foreign and Commonwealth Office in Westminster, London, for a meeting with Foreign Secretary Dominic Raab.
Cardboard cutout of Northampton Town fan Harry Dunn who died in a car crash last August is seen in the stands during the Sky Bet League Two play-off semi final first leg match at the PTS Academy Stadium, Northampton.
Business Secretary Andrea Leadsom and Harry Dunn's mother Charlotte Charles outside South Northamptonshire Council offices in Towcester, after they met following the news that the US have refused to extradite Anne Sacoolas who is charged with causing his death in a road collision outside RAF Croughton in August last year.
A motorbike convoy makes it's way through the village of Charlton after following Harry Dunn's last ride as a tribute to the teenager who died when his motorbike was involved in a head-on collision near RAF Croughton, in Northamptonshire in August.
Charlotte Charles (left), the mother of Harry Dunn, Bruce Charles, and Tracey Dunn speak to media after a motorbike convoy followed Harry Dunn's last ride through Brackley as a tribute to the teenager who died when his motorbike was involved in a head-on collision near RAF Croughton, Northamptonshire, in August.
Chief Constable of Northamptonshire Police, Nick Adderley (centre) and Superintendent Sarah Johnson speaking during a press conference at Northamptonshire Police HQ at Wootton Hall Park, Northampton about the death of Harry Dunn.
Harry Dunn's Auntie Katie Grant (right) and Nicola Watson tie a Green Ribbons which are being made be members of the village of Charlton, Oxfordshire in memory of 19-year-old Harry Dunn who died while out riding his Green motorcycle near RAF Croughton.
Please Drive on Left signs and arrows have been placed on the B4031 road outside RAF Croughton, in Northamptonshire, where Harry Dunn, 19, died when his motorbike was involved in a head-on collision in August.
(Left to right) Bruce Charles and Charlotte Charles (Harry's mother), lawyer Radd Seiger, Tim Dunn (Harry's father) and Tracey Dunn at a press conference at the Parker New York Hotel in New York, US, where Charlotte Charles said that Anne Sacoolas, the American woman suspected of causing her son's death, should be brought back to the UK to face justice.
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In 1995, the UK agreed to an American request to include staff at RAF Croughton on the diplomatic list, but asked the US to waive the immunity of administrative and technical staff in relation to "acts performed outside the course of their duties".
The FCDO says that waiver only applied to staff at RAF Croughton and not their family members, meaning Anne Sacoolas did have immunity at the time of the crash.
But, at a remote hearing on Wednesday, Sam Wordsworth QC – representing Ms Charles and Mr Dunn – said Sacoolas had "no duties at all" at the base and therefore "never had any relevant immunity for the US to waive".
Mr Wordsworth told the court that, under the 1995 agreement, "the US agreed to waive the immunity of the administrative and technical staff from criminal jurisdiction in respect of acts performed outside the course of their duties".
The barrister added: "It follows that administrative and technical staff at RAF Croughton were only ever entitled to a limited immunity.
"Mrs Sacoolas could and did only derive her immunities and privileges from Mr Sacoolas in circumstances where no immunity from criminal jurisdiction was conferred on Mr Sacoolas in respect of acts performed outside the course of his duties.
"Mrs Sacoolas, likewise, had no such immunity, hence she was not immune with respect to the criminal proceedings at issue in this case."
Geoffrey Robertson QC – also representing Ms Charles and Mr Dunn – earlier said the FCDO "took upon itself the authority to resolve the question of immunity and ultimately and unlawfully decided to accept the US embassy's decision that Anne Sacoolas had immunity".
He said that decision "obstructed the police by preventing any effective further progress in its investigation into Harry's death and likely prosecution of Anne Sacoolas".
Mr Robertson argued the FCDO "tacitly accepted the Sacoolas family's departure from the UK", referring to a text message sent to a US embassy official on September 14 2019 – a day before Sacoolas and her family left the UK.
The message read: "I think that now the decision has been taken not to waive (immunity), there's not much mileage in us asking you to keep the family here.
"It's obviously not us approving of their departure but I think you should be able to put them on the next flight out."
Mr Robertson added that it would be "absurd" for family members to have "greater privileges and immunities" than the staff at RAF Croughton "from whom their immunity flows".
In written submissions, the FCDO's barrister, Sir James Eadie QC, said: "As a matter of international and domestic law, Mrs Sacoolas automatically had diplomatic immunity as the spouse of a member of the administrative and technical staff of the US mission."
He argued the FCDO "plainly did not obstruct the police investigation", adding: "On the contrary, the Secretary of State sought to assist the police investigation, including by requesting a waiver of Mrs Sacoolas's immunity."
Sir James told the court the US "expressly waived the immunity from the UK's criminal jurisdiction of 'employees' or 'staff members'", but "at no point is there a waiver of the immunity enjoyed by the families of such individuals".
He also said the FCDO officials had "objected in strong terms" to Sacoolas leaving the UK, and "repeatedly emphasised" that the department "wanted the Sacoolas family to co-operate with the UK authorities".
Sir James added that the person who sent the text message to a US embassy official last September "had the previous day informed the recipient in person of the UK's strong objections to the US's intended course of action, but it was clear that there was no realistic prospect of convincing the US to change its approach".
The hearing before Lord Justice Flaux and Mr Justice Saini is expected to conclude on Thursday.