The US has turned down an extradition request for one of its citizens to be charged over the death of motorcyclist Harry Dunn.
Mr Dunn, 19, died after a crash in Northamptonshire in August which led to the suspect, Anne Sacoolas, leaving for the US under diplomatic immunity.
The Home Office started extradition proceedings on charges of causing the death by dangerous driving.
A spokeswoman said the decision by the US “appears to be a denial of justice”.
The US Secretary of State Mike Pompeo turned down the extradition request in an email to the UK Government, family spokesman Radd Seiger said.
He 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 the suspect ever returning to the UK was very low.
Mr Seiger said “no reason” was given by Mr Pompeo in rejecting the extradition request.
He also said that 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 Sicoolas will be coming back one day,” he said.
“This is far from over.”
A spokeswoman for the Home Office said: “We are disappointed in this decision which appears to be a denial of justice.
“We are urgently considering our options.”
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.”