A judge's ruling in the Alexandria District Court in Virginia has taken the Dunn family a step closer to a legal showdown with suspect Anne Sacoolas, 18 months on from the 19-year-old's death.
Should there be no settlement in the case, the next legal step would be a “deposition”, in which Ms Sacoolas and her husband would be forced to provide their account of events outside of court.
Mr Dunn's mother, Charlotte Charles, father Tim Dunn and twin brother Niall, would have the option to attend the deposition.
The US government asserted diplomatic immunity on behalf of 43-year-old Ms Sacoolas following the road crash which killed Mr Dunn outside RAF Croughton in Northamptonshire in August 2019.
She was charged with causing death by dangerous driving, but an extradition request submitted by the Home Office was rejected by the US State Department in January last year.
On Wednesday, Judge Thomas Ellis ruled the Dunn family could proceed with their civil claim against both Mr and Ms Sacoolas - allowing a claim of “vicarious liability” to be brought against the suspect's husband.
The Virginia State law of vicarious liability means Mr Sacoolas could be liable for the teenager's death by allowing his wife to use the car which killed him.
The judge told Ms Sacoolas's lawyer, John McGavin, he “would probably have read the case a little bit differently” if the pre-trial procedure of “discovery”, or the compilation of evidence, was “easier to come by”.
Reacting to the judge's ruling, the Dunn family's spokesman Radd Seiger told the PA news agency: “Harry's family are very pleased at the ruling today and that their claims have been allowed to proceed in full.
“They are also pleased that Mr Sacoolas will also have to be deposed so that they can learn the full account of what happened on the night Harry died.
“The Sacoolases and their insurers, the United Services Automobile Association (USAA), have sought to deny Harry's family justice at absolutely every turn along the way. Common sense has prevailed tonight.”
Mr Seiger continued: “The Dunn family are entitled to pursue their legal rights to the fullest extent possible, ultimately to be determined by the court at trial.
“It is time to get on with delivering justice to this family and to put these petty attempts to deprive them of their rights behind us.
“The USAA have done themselves a huge disservice in their handling of the claims to date.”
In February, Judge Ellis ruled the substantive claim against Ms Sacoolas can go ahead in the US, despite the suspect's motion to dismiss it.
While making the application, lawyers acting on behalf of Ms Sacoolas made admissions about her employment at the time of the road crash which killed Mr Dunn, saying she and her husband worked for the US state department and “fled” the UK due to “issues of security”.
Judge Ellis allowed the case to remain in the US and dismissed Ms Sacoolas's application against the substantive civil claim, citing her “refusal” to return to the UK.
Handing his judgment down last month, the judge said: “While it is commendable that defendant Anne Sacoolas admits that she was negligent and that her negligence caused Harry Dunn's death, this does not equate acceptance of responsibility.
“Full acceptance of responsibility entails facing those harmed by her negligence and taking responsibility for her acts where they occurred, in the United Kingdom.”
During a short hearing on Wednesday, Judge Ellis told the parties the case could be settled outside of court if both sides agreed to have those discussions.
Before the hearing, the prime minister said the foreign secretary had raised the case with the US secretary of state.
The PM earlier confirmed to the Dunn family's constituency MP, Andrea Leadsom, that the UK government continues to raise the case “at the highest level”.
Asked at Prime Minister's Questions if he could try to persuade President Joe Biden to deliver justice for Mr Dunn, Boris Johnson said: “She's completely right to continue to raise the case of Harry Dunn and we sympathise deeply with his family.
“It's a case that we continue to raise at the highest level and I know that my Right Honourable friend the foreign secretary has only just raised it now with Tony Blinken, the US secretary of state.”
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