Seven weeks after Harry Dunn was killed in a car crash involving the wife of an American diplomat, his parents have flown to the US to seek justice.
Anne Sacoolas was granted diplomatic immunity after the accident in Northamptonshire on 27 August and left the UK, prompting an international dispute between London and Washington over what should happen to her.
So far, the Foreign Office has had no success in its efforts to persuade the White House to send her back to the UK, and few signs exist that Donald Trump would sanction any move that could see her face trial abroad.
Sacoolas, 43, has offered an apology over the fatal crash outside RAF Croughton, but has been told by Harry's mother Charlotte Charles that "sorry doesn't cut it".
With the US and UK at loggerheads, what are the options available to Ms Charles and Tim Dunn in their efforts to secure justice for their son?
In the first scenario, Sacoolas could come back voluntarily to assist police with their inquiries.
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Northamptonshire Police is handling the case, and the Foreign Office has said the force is free to take it forward because Sacoolas lost her diplomatic immunity after leaving the UK.
But Michael McParland QC, who is a US attorney, told Sky News her lawyers are likely to have advised her not to come back to be questioned by officers.
He added: "I have a horrible feeling if she did that she would have her American lawyer sat next to her saying 'don't answer those questions because you might incriminate yourself'."
Amy Jeffress, who is representing Sacoolas, has criticised "inaccurate" reporting about her client, insisting that she had already "fully co-operated with the police and the investigation".
Detective Superintendent Sarah Johnson, of Northamptonshire Police, confirmed to Sky News: "The driver that we are seeking did engage with us after the original collision."
Mr McParland added: "Is she going to come back voluntarily? I would suspect the answer to that is 100% no."
If Sacoolas is charged with an offence in the UK, then the British government could apply for her extradition – an extradition treaty has existed between the US and the UK since 2007.
Mr McParland said: "Northamptonshire Police will have to decide if there is sufficient evidence to press charges, and pass it on to Crown Prosecution Service, who will decide if there is and if it is in the public interest.
"And then the question would shift to whether there can be an application for extradition."