The US has turned down an extradition request for a woman who is to be charged with causing the death of teenage motorcyclist Harry Dunn.
Mr Dunn, 19, died after a crash in Northamptonshire in August which led to the suspect Anne Sacoolas, the wife of a US intelligence officer, leaving for the US under diplomatic immunity.
The Home Office launched extradition proceedings earlier this month.
A spokeswoman said the decision by the US “appears to be a denial of justice”.
Washington, however, said granting the request would “render the invocation of diplomatic immunity a practical nullity”.
The US Secretary of State Mike Pompeo turned down the extradition request in an email to the UK Government on Thursday evening, the Dunn family spokesman Radd Seiger said.
Mr Seiger said the family would react fully to the news on Friday morning, adding “the fight goes on” for justice for their son.
The family’s constituency MP Andrea Leadsom is due to meet the US ambassador Woody Johnson in London on Friday to discuss the case.
When the Home Office started extradition proceedings the US State Department said the request would be “highly inappropriate”.
It insisted Ms Sacoolas’ status at the time of the crash meant she had diplomatic immunity.
Prime Minister Boris Johnson previously said the chance of Ms Sacoolas, who is to be charged with causing the death by dangerous driving, ever returning to the UK was very low.
Mr Dunn died after his motorbike was in collision with a car owned by Mrs Sacoolas.
The crash happened outside RAF Croughton where Mrs Sacoolas’ husband Jonathan worked as an intelligence officer.
The 42-year-old left the UK and returned to her native US, claiming diplomatic immunity.
In a statement released on behalf of the suspect after she was charged in December, Mrs Sacoolas’s lawyers said: “Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident.”
Mr Seiger said “no reason” was given by Mr Pompeo in rejecting the extradition request.
He also said the family had expected the decision, adding: “We knew this day was coming. It is no surprise.”
However, he said the family would continue its campaign and that the extradition request “hangs there forever”.
“I can assure you that Anne Sacoolas will be coming back one day,” he said. “This is far from over.”
The Home Office said it was “disappointed in this decision which appears to be a denial of justice”.
“We are urgently considering our options,” a spokeswoman added.
A statement from the US State Department said: “At the time the accident occurred, and for the duration of her stay in the UK, the US citizen driver in this case had immunity from criminal jurisdiction.
“If the United States were to grant the UK’s extradition request, it would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent.”