The mother of a teenager killed in a car crash involving the wife of a US diplomat has urged her "as a mum" to return to the UK for questioning.
Harry Dunn, 19, died when his motorbike collided with a car near RAF Croughton in Northamptonshire on 27 August.
The diplomat's wife, who has diplomatic immunity, left the UK despite telling police that she had no plans to.
Mr Dunn's mother, Charlotte Charles, told the BBC the family had been left "utterly devastated" by his death.
Foreign Secretary Dominic Raab said he has urged the US embassy to reconsider after the State Department said that diplomatic immunity is "rarely waived".
"I have called the US ambassador to express the UK's disappointment with their decision," he said.
Mrs Charles told the BBC's PM programme: "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 [Mr Raab] can try to get through to her.
"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."
Under the 1961 Vienna Convention, diplomats and their family members are immune from prosecution in their host country, so long as they are not nationals of that country.
However, their immunity can be waived by the state that has sent them - in this case, the US.
Analysis by BBC diplomatic correspondent James Landale
There are more than 22,500 people in the UK who hold diplomatic immunity and most do not break the law.
But if a diplomat is guilty of an egregious breach, there are some things that a host country can do.
In a written Parliamentary answer in October 2017, then Foreign Secretary Boris Johnson said: "The FCO does not tolerate foreign diplomats breaking the law.
"When instances of alleged criminal conduct are brought to our attention by the police, we ask the relevant foreign government to waive diplomatic immunity where appropriate.
"For the most serious offences, and when a relevant waiver has not been granted, we seek the immediate withdrawal of the diplomat."
The problem here is that the US do not appear to have granted a waiver for this particular diplomatic spouse.
Instead, they have removed her from the UK before the British government could threaten to remove her itself if she did not submit to questioning.
As such, the US appears to have calculated that protecting the woman from identification, questioning and possible prosecution was more important than the potential risk to UK-US relations.
This is further evidence the adjective "special" should rarely be used to describe the alliance between both countries.
Supt Sarah Johnson said that the suspect "engaged fully" following the incident near RAF Croughton, a US Air Force communications station, and that she "had previously confirmed... that she had no plans to leave the country in the near future".
"The force is now exploring all opportunities through diplomatic channels to ensure that the investigation continues to progress," she said.
The US Embassy in London confirmed the diplomat's family had left the UK, but it could not confirm the identity of the people involved in the incident "due to security and privacy considerations".
The US State Department said it was in "close consultation" with British officials, but could not comment on "private diplomatic conversation" with the British government.
"We express our deepest sympathies and offer condolences to the family of the deceased in the tragic August 27 traffic accident involving a vehicle driven by the spouse of a U.S. diplomat assigned to the United Kingdom," a spokesperson said.
"Any questions regarding a waiver of 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."
Andrea Leadsom MP, Secretary of State for Business, Energy and Industrial Strategy, tweeted that she had met Mr Dunn's family, who she described as "heartbroken".
I met Harry’s family yesterday. They are totally heartbroken. We have to get proper justice for Harry and closure for his family. https://t.co/WPk1OOpVKM— Andrea Leadsom MP (@andrealeadsom) October 5, 2019
Angela Rayner MP, shadow secretary of state for education, tweeted that the family have been "wronged".
If we stand for anything it’s the common rules of fairness. Harry & his family have been wronged. You have to act. The diplomatic rules should not be used in this case. @DominicRaab @POTUS @10DowningStreet @BorisJohnson #Justice4harry— Angela Rayner 🌈 (@AngelaRayner) October 5, 2019