HOME Secretary Priti Patel has sent the US an extradition notice for Anne Sacoolas to face UK justice over Harry Dunn's death.
The teen's family spokesman said they are "relieved" in the latest development in their battle to bring the diplomat's wife back.
He added: "There is no doubt in my mind and... there never has been.
"Whether it's today, or tomorrow, or in five years' time or in 10 years, Anne Sacoolas will come back, she has to come back."
In December the American was charged by the CPS over the 19-year-old's death - after his family vowed to bring him justice.
He died after being hit by a car the 42-year-old was reportedly driving outside RAF Croughton in Northamptonshire in August last year.
She left the UK and claimed diplomatic immunity shortly after his death - her husband is thought to be a US intelligence officer.
US STATE DEPARTMENT RESPONSE
A spokeswoman said: "This was a tragic accident, a young man has lost his life, and his family is grieving. No-one could hear about this tragic accident and not feel incredible sadness over this loss. The president, the secretary of state, the US ambassador in London, and others in our government have all expressed sincere condolences to the Dunn family for this tragedy.
"The United States has been clear that, at the time the accident occurred, and for the duration of her stay in the UK, the driver in this case had status that conferred diplomatic immunities.
"The Foreign Secretary stated the same in Parliament. 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.
"The United States has a strong law enforcement relationship with the UK and, in particular, a strong track record of close cooperation on extradition matters. However, under the circumstances of this case, we strongly believe that an extradition request would be highly inappropriate.
"Since the accident occurred, the United States has been engaged with the UK, and we have been transparent about our positions on legal, diplomatic, and other matters. We continue to be engaged with the UK government to look at options for moving forward."
Mr Seiger added tonight: "This will not of course bring Harry back, but in the circumstances of all that this family have been through, they are pleased with the development and feel that it is a huge step towards achieving justice for Harry and making good on the promise that they made to him on the night he died that they would secure justice for him.
"Despite the unwelcome public comments currently emanating from the US administration that Anne Sacoolas will never be returned, Harry's parents, as victims, will simply look forward to the legal process unfolding, as it must now do, confident in the knowledge that the rule of law will be upheld.
"They will simply take things one step at a time and not get ahead of themselves. However, no one, whether diplomat or otherwise, is above the law."
BRING HER BACK
Confirming the extradition request, a spokesman for the Home Office said: "Following the Crown Prosecution Service's charging decision, the Home Office has sent an extradition request to the United States for Anne Sacoolas on charges of causing death by dangerous driving.
"This is now a decision for the US authorities."
Business Secretary, Andrea Leadsom, has written to the Prime Minister to request a meeting between him and Harry Dunn's parents.
Mrs Leadsom, the family's constituency MP in South Northamptonshire, said in a letter to Boris Johnson on Thursday: "I have met Harry's parents a number of times and they are understandably finding it very difficult to start to grieve for their son fully until there is some closure in the case."
The US State Department and Mrs Sacoolas' legal teams have said she won't willingly return to the UK.
And one of her lawyers said a long jail sentence was "unworthy of someone of her standing".
In December Harry's parents Charlotte Charles and Tim Dunn were told she is no longer immune from prosecution in a letter from the Foreign Secretary Dominic Raab.
Anne Sacoolas statement in full
"Anne is devastated by this tragic accident and continues to extend her deepest condolences to the family," the statement said.
"Anne would do whatever she could to bring Harry back. She is a mother herself and cannot imagine the pain of the loss of a child. She has cooperated fully with the investigation and accepted responsibility.
"We have been working with the UK authorities in an effort to resolve this matter. Today we were informed that the Crown Prosecution Service has decided to bring a prosecution for wrongful death by dangerous driving, and the UK government may seek extradition.
"This was an accident, and a criminal prosecution with a potential penalty of fourteen years imprisonment is simply not a proportionate response.
"We have been in contact with the UK authorities about ways in which Anne could assist with preventing accidents like this from happening in the future, as well as her desire to honour Harry's memory.
"We will continue that dialogue in an effort to move forward from this terrible tragedy. But Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident."
Charlotte said at the time: "I'm going to go and buy a Christmas tree and decorate it in green lights like so many of our supporters have done.
"I never thought I'd buy a Christmas tree this year so thank you."
While dad Tim added: "We set out so long ago that we believed and we believed and we've done it. This is it. It's amazing, absolutely amazing.
"Justice. Whatever happens now doesn't matter, we got we wanted."
But the US State Department said it fears “it will not bring a resolution closer” and said it would be an "egregious abuse" to deport her back to Britain.
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A spokesperson added: "The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent."
They have the power to refuse extradition if they believe Sacoolas still has diplomatic immunity and if the alleged offence is not a crime in both countries.
And Sacoolas also released a statement saying she would not "voluntarily" return to Britain to face a "potential jail sentence for what was a terrible but unintentional accident".