Harry Dunn’s alleged killer was never entitled to immunity from prosecution in the UK, the High Court has heard.
Mr Dunn, 19, died when his motorbike was in a crash with a car near RAF Croughton, Northamptonshire, on 27 August last year.
The suspect, Anne Sacoolas, later left for the USA citing diplomatic immunity.
She was charged with causing death by dangerous driving in December but an extradition request was denied in January.
Mrs Sacoolas, 43, whose husband Jonathan worked as a technical assistant at the base, was allegedly driving on the wrong side of the road when she hit Mr Dunn.
His parents, Charlotte Charles and Tim Dunn, claim the Foreign, Commonwealth and Development Office (FCDO) wrongly decided Mrs Sacoolas had diplomatic immunity and unlawfully obstructed Northamptonshire Police’s investigation.
In 1995, the UK agreed 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 Mrs Sacoolas did have immunity at the time of the crash.
But, at a remote hearing, Sam Wordsworth QC – representing the parents – said Mrs Sacoolas had “no duties at all” at the base and therefore “never had any relevant immunity for the US to waive”.
He told the court that, under the 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”.
“It follows that administrative and technical staff at RAF Croughton were only ever entitled to a limited immunity.”
He said as “no immunity from criminal jurisdiction was conferred on Mr Sacoolas in respect of acts performed outside the course of his duties” it followed that his wife was not beyond prosecution either.
“Hence she was not immune with respect to the criminal proceedings at issue in this case,” he said.
Geoffrey Robertson QC – also representing the parents – earlier said the FCDO “tacitly accepted the Sacoolas family’s departure from the UK”.
He referred to a text message sent to a US embassy official on 14 September 2019 – a day before Mrs Sacoolas and her family left for the USA.
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.”
In written submissions, the FCDO’s barrister Sir James Eadie QC said “Mrs Sacoolas automatically had diplomatic immunity as the spouse of the administrative and technical staff of the US mission”.
He denied claims the FCDO had obstructed the police investigation.
He said officials had “objected in strong terms” to Mrs Sacoolas leaving the UK, and “repeatedly emphasised” that the department “wanted the Sacoolas family to co-operate with the UK authorities”.
The hearing before Lord Justice Flaux and Mr Justice Saini is expected to conclude on Thursday.