The parents whose son died in a crash involving a diplomat's wife have launched an appeal for witnesses to who may have seen her as she left to return to the US.
Harry Dunn, 19, died when his motorbike crashed into a car outside an RAF base in August.
Anne Sacoolas, 43, the wife of a US diplomat, was granted diplomatic immunity after the crash.
The teenagers' parents, Charlotte Charles and Tim Dunn, have now taken their fight for justice to the US.
They plan to visit both New York and Washington DC to whip up support from Americans and put pressure on the US government to "do the right thing".
Before they left the UK, family spokesman Radd Seiger said they were appealing for witnesses who saw Mrs Sacoolas leave the country for the US.
He said: "Anne Sacoolas left England to return to the USA following the road traffic collision outside RAF Croughton on August 27.
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"The United States government has claimed she has the benefit of diplomatic immunity and has refused to grant the waiver sought by the British government.
"I would urge anyone, on either side of the Atlantic, who has any information relating to Mrs Sacoolas's return to the United States, whether before, during, or after her departure, to please come forward."
Boris Johnson said on Friday that America was "ruthless" in safeguarding Mrs Sacoolas and it was "very reluctant" to allows its citizens to go on trial abroad.