An extradition request for the US woman charged with causing the death of teenage motorcyclist Harry Dunn by dangerous driving has been turned down by Secretary of State Mike Pompeo, the family spokesman has said.
The 19-year-old's parents were informed of Mr Pompeo's decision in a phone call with their constituency MP Andrea Leadsom on Thursday.
Mr Dunn was killed when his motorbike crashed into a car outside a US military base in Northamptonshire on August 27 last year.
Anne Sacoolas, 42, was charged with causing his death by dangerous driving by the Crown Prosecution Service in December.
Mrs Sacoolas was granted diplomatic immunity following the crash and was able to return to her home country, sparking an international controversy.
Boris Johnson previously said the chance of the suspect ever returning to the UK was very low.
The State Department on Thursday refused to grant the extradition request and said that doing so would set an "extraordinarily troubling precedent".
"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," a spokesperson for the department said.
"The United States government again expresses its sincere condolences and sympathy to the Dunn family for the loss of their son."
Mr Dunn's family said they would react fully to the news on Friday morning, but said "the fight goes on" for justice for their son.
In response to the extradition request being rejected, 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."