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, claimed 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”.
But, in a High Court judgment delivered on Tuesday, Lord Justice Flaux and Mr Justice Saini said: “Our conclusion is that Mrs Sacoolas enjoyed immunity from UK criminal jurisdiction at the time of Harry’s death.”
The judges also rejected Mrs Charles and Mr Dunn’s claim the FCDO “usurped” Northamptonshire Police’s investigation into their son’s death, finding officials “sought to assist rather than obstruct Northamptonshire Police in their investigation”.
Speaking about the family’s next steps, Ms Charles said that she hoped Joe Biden would “show us that the US have morals”.
When asked how she felt about the ruling, she said that it was "another blip".
“Just that it’s another blip. It’s just another hurdle that they put in the way", she said.
“We knew that this was a possibility but we always have a plan B.Our team, our lawyers were very, very confident right from the very beginning of this campaign that she did not have immunity at the time of the crash when she killed Harry.
“They are still as confident as ever with that opinion. We have the Director of Public Prosecutions behind us with that as well.If anything, we are more confident than what we were that she did not have immunity.”
Mrs Charles and Mr Dunn’s case centred on a 1995 agreement between the UK and the US, granting immunity to administrative and technical staff at RAF Croughton, which the US waived in relation to “acts performed outside the course of their duties”.
At a hearing earlier this month, their lawyers 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”.
Sam Wordsworth QC told the court that Sacoolas had “no duties at all” at the base and therefore “never had any relevant immunity for the US to waive”.
But Lord Justice Flaux and Mr Justice Saini found Sacoolas had diplomatic immunity “on arrival in the UK” under the Vienna Convention on Diplomatic Relations (VCDR) which had not been “expressly waived”, meaning she “had immunity at the time of Harry’s death”.
The judges said: “We do not come this conclusion with any enthusiasm for the result, but it is compelled by the operation of the VCDR.”
Foreign Secretary, Dominic Raab, said his thoughts were with Harry Dunn’s family.
“My thoughts today are with Harry’s family", he said.
"While this judgment makes clear the Foreign Office acted properly and lawfully throughout, I appreciate that won’t provide any solace to the family in their search for justice. We stand with them, we’re clear that Anne Sacoolas needs to face justice in the UK, and we will support the family with their legal claim in the US.”