The mother of a teenager killed in a road crash has appealed for a US diplomat's wife linked to the accident to return to the UK, as American officials warned "immunity is rarely waived".
Harry Dunn, 19, of Charlton, Banbury, died after his motorbike collided with a car close to RAF Croughton, Northamptonshire, on August 27.
Northamptonshire Police confirmed a 42-year-old American woman who was being treated as a suspect in their investigation has left the country.
Mr Dunn's family have urged her to return to the UK as British authorities who asked for her immunity to be removed had their appeals rejected.
Speaking to the PM Programme on BBC Radio 4, Mr Dunn's mother Charlotte Charles said: "We're really hoping to try to get her back, from me as a mum, to her, as a mum, you just hope that he [Foreign Secretary Dominic Raab] can try to get through to her, that we don't wish her any ill harm, but we don't understand how she can just get on a plane and leave our family just utterly devastated.
"If we don't get any luck over here, then we will go over there."
The US Embassy confirmed the incident involved "a vehicle driven by the spouse of a US diplomat assigned to the United Kingdom" and that the family has left the UK.
An embassy spokesman said: "We express our deepest sympathies and offer condolences to the family of the deceased in this tragic traffic accident.
"Any questions regarding a waiver of the immunity with regard to our diplomats and their family members overseas in a case like this receive intense attention at senior levels and are considered carefully given the global impact such decisions carry; immunity is rarely waived.
"The US Embassy has been and will continue to be in close contact with appropriate British officials."
According to the Vienna Convention on Diplomatic Relations, families of diplomats are granted immunity from arrest. However, the sending state is able to issue a waiver of immunity.
Foreign Secretary Dominic Raab has since called the US ambassador to ask the embassy to "reconsider" its decision.
Mr Dunn's parents are now hoping to meet with the Foreign Secretary next week after meeting with South Northamptonshire Tory MP Andrea Leadsom on Friday.
The Secretary of State for Business, Energy and Industrial Strategy met with Mr Dunn's family and said they are "totally heartbroken".
She added: "We have to get proper justice for Harry and closure for his family."
Family spokesman Radd Seiger said it is understood British authorities asked their US counterparts for immunity to be waived "several times".
He said: "They've been told the answer is no, we've learnt via the police. The answer has come back as no."
Mr Seiger said Mrs Charles is due to speak to American TV broadcasters this weekend in the hope of raising the family's cause in Washington.
He added that Mr Dunn's parents are hoping to meet with Mr Raab next week.
Mr Seiger previously said the family are "utterly devastated" and "not coping well at all" following the teenager's death.
He added: "If that weren't bad enough, they now have a situation not of their making where the American Government has whisked this American citizen back to the US before they could get some justice."
Speaking to Sky News about her son, Mrs Charles added: "Everyone loved him, we're utterly broken inside and out, everything hurts day and night, it's an effort to do anything, I ache from it, every limb, every internal organ hurts.
"We don't know how we can start to grieve for him.
"We have nothing. No justice. We have nothing to put our minds at rest that she's even remorseful."