The United States has criticised the UK's request to extradite an American accused of killing motorcyclist Harry Dunn, calling it "highly inappropriate".
Mr Dunn, 19, died after being hit by a car allegedly driven by suspect Anne Sacoolas, who left the country for the US claiming diplomatic immunity.
The Home Office submitted a request on Friday to extradite her to the UK.
Dunn family spokesman Radd Seiger said she will "100% be coming back".
"I have no doubt in my mind, the only thing I can't tell you is when," he told BBC Breakfast.
"This campaign won't stop until Anne Sacoolas is back in the UK facing the justice system. There is no celebration and until she is back, we won't rest.
"This lady is accused of taking Harry's life, then fleeing the country. No-one is above the law in modern society. You don't get to move to a country, break a law in that country and then leave."
Mr Seiger said that under the circumstances, the family was "really pleased" the UK authorities had taken the "huge step towards justice", but if the Trump administration was to ignore or reject the request, it would be re-presented should another administration come into power.
Can Anne Sacoolas be extradited?
The extradition request is sent via the British Embassy to the US State Department.
A lawyer will then decide whether it falls under the dual-criminality treaty, where the alleged offence is a crime in both countries and carries a prison sentence of at least a year.
The maximum sentence for causing death by dangerous driving is 14 years' imprisonment, although this is usually reserved for the most serious cases.
The US may reject the request for extradition, arguing that Mrs Sacoolas is still entitled to diplomatic immunity.
The crash happened outside RAF Croughton, where Mrs Sacoolas' husband Jonathan worked as an intelligence officer. Mr Dunn died after his motorbike was in collision with a car owned by Mrs Sacoolas.
The 42-year-old left the UK shortly after the crash on 27 August and returned to the US, prompting a justice campaign by the teenager's parents, Charlotte Charles and Tim Dunn.
Mrs Sacoolas was charged in December by the Crown Prosecution Service with causing death by dangerous driving and the Home Office submitted its extradition request to the US Department of Justice.
A spokeswoman for the US State Department said: "It is the position of the United States government that a request to extradite an individual under these circumstances would be an abuse.
"The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent."