The family of teenage crash victim Harry Dunn are launching legal action against the Government over the way it handled the death of their son in Northamptonshire.
Announcing plans for legal action against the Foreign Office on Thursday, spokesman for Mr Dunn's family Radd Seiger told Sky News: "We will shortly be issuing a letter of claim which is a prelude to a judicial review.
"We are absolutely clear that the Foreign Office's decision to advise Northamptonshire Police that Anne Sacoolas had the benefit of diplomatic immunity was unlawful. We will be seeking a judicial review of that decision to have it quashed."
The 19-year-old motorcyclist died when his motorbike crashed into a car driven by US woman Mrs Sacoolas outside RAF Croughton in Northamptonshire on August 27.
Mrs Sacoolas left the UK after being granted diplomatic immunity.
The family have also referred Northamptonshire Police to the Independent Office for Police Conduct over the investigation into the death of their son, Mr Seiger said.
Mr Seiger continued: “It’s absolutely clear that Mrs Sacoolas was involved in a very serious collision that ended in the loss of Harry’s life.
“We are clear she’s admitted her culpability, that she… committed a very serious crime that night.
“At the very least careless, causing death by careless driving and at the worst causing death by dangerous driving. She is a fugitive from this country and she is on the run.
“We appeal to her to come back to this country and face the music.”
Mrs Charles and Mr Dunn travelled to the US last week in an attempt to put pressure on authorities to return Mrs Sacoolas to the UK.
The visit included talks at the White House with President Donald Trump, during which the family was told that Mr Dunn's alleged killer was in a nearby room and prepared to meet them. They refused the meeting, insisting such an encounter take place in Britain.
Speaking on Thursday, Mr Seiger questioned if Mrs Sacoolas was entitled to diplomatic immunity through family links, adding: “This person, whoever he is, was not a diplomat and therefore does not have the benefit of diplomatic immunity and nor does his family.
“The other thing that we are going to be doing shortly is, unfortunately, referring Northamptonshire Police to the Independent Office for Police Conduct.
“Although they (Northamptonshire Police) have not disclosed all the information this family are entitled to, we have deep concerns about the manner in which this investigation was conducted, and simply adding insult to injury to this family at their darkest hour.”