Police have warned the spokesman for Harry Dunn’s family to be more careful about his comments on the teenager’s death, as officers prepare to travel to the US to question the American diplomat’s wife alleged to have killed him.

The chief constable of Northamptonshire Police urged Radd Seiger to “exercise constraint” [sic] in his public statements after he attacked the force’s handling of the investigation as “unacceptable and unforgivable”.

The rebuke came as it emerged Anne Sacoolas, a US national who claimed diplomatic immunity to leave the UK following the car crash which killed 19-year-old Dunn in August, had asked to be interviewed under caution by British detectives.

Chief constable Nick Adderley said her lawyers had “clearly stated that the suspect wants to be personally interviewed by officers from Northamptonshire Police in order for them to see her and the devastation this has caused her and her family.”

Ms Sacoolas declined to provide a written statement and “requested to be interviewed by British police officers, under caution, in the United States,” Mr Adderley told a press conference on Tuesday. He added: “The suspect has cooperated fully with the police and with the authorities.”

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Officers are awaiting the necessary visas before travelling to the US to interview Ms Sacoolas, who has previously admitted she was driving on the wrong side of the road when her car collided with Dunn’s motorbike near RAF Croughton, a US-run military base in Northamptonshire, on 27 August.

The American, whose husband is an intelligence officer, flew out of the country on a US air force plane on 15 September without giving her account of the crash to police.

Dunn’s parents, Charlotte Charles and Tim Dunn, have demanded answers from detectives, the UK Foreign Office and the American government about why Ms Sacoolas was allowed to leave the UK.

Mr Seiger, their spokesman, claimed on Monday that police “knew weeks before the family were told that Mrs Sacoolas had left the country”. He said authorities’ failure to keep the Dunn’s parents informed “only compounds the family’s pain and misery”.

“They feel totally abandoned by the system and were left with no choice but to take their case to the court of public opinion,” he added.

Speaking in Tuesday’s press conference, Mr Adderley urged Mr Seiger to “exercise constraint [sic] in his commentary as the case moves forward”.

“I understand the emotion and I understand the anxiety,” he told reporters. “I also understand the will and the want for answers to questions the family are asking.”

He added: “From the police’s point of view we have to make sure we maintain the integrity of all of the evidence and that it is not skewed by [the diplomatic immunity debate] to ensure a fair trial should we get to an extradition position and she comes back to the UK. We don’t want to jeopardise that by not affording her a fair trial.”

Tin Dunn and Charlotte Charles speak to media in the US (Reuters)

Asked if he accepted his force had caused unnecessary trauma to Dunn’s family by delaying informing them that Ms Sacoolas had left the country, the chief constable said it was “very much a judgment call”.

“If Charlotte and Tim have been upset by that delay that is something I am prepared to apologise for – but it is a decision I would still support,” he added.

Foreign secretary Dominic Raab has admitted the government asked police to delay informing Dunn’s parents that Ms Sacoolas had fled Britain, fuelling the family’s fears of “a cover-up on both sides of the Atlantic”.

Dunn’s parents have welcomed the news that police are planning to interview Ms Sacoolas, although his father said: “Good luck trying to find her.”

Lawyer for family of Harry Dunn says they were 'ambushed' by President Trump when he tried to spring surprise meeting with woman involved in crash

The pair had been scheduled to speak to Mr Adderley on Wedesday afternoon but pulled out of the meeting after it became clear he would not answer “a series of key questions” the family had prepared. The chief constable contacted the family to “provide clarity” that the meeting would only be a “private and personal visit to express condolences”.

Mr Seiger later tweeted: “The time for condolences has long since passed and the answers to the many questions about Anne Sacoolas’s departure and next steps are long overdue!”

Mr Adderley, responding to criticism on Twitter, said there were “many questions being asked of the police which would be wholly inappropriate to speculate on”.

On Tuesday Ms Charles and Mr Dunn are to meet with shadow foreign secretary Emily Thornberry, who is expected to call for a parliamentary inquiry into the teenager’s death.

Mr Raab told the Commons on Monday he had commissioned a review into immunity arrangements for US personnel and their families at the RAF Croughton annex in light of the case.

He insisted there were “no barriers to justice being done” and that Ms Sacoolas’s diplomatic immunity had ”clearly ended” when she left Britain. The foreign secretary said it was up to police and the Crown Prosecution Service to determine which steps to take next, telling MPs he was “not aware of any obstacle” under the UK/US Extradition Treaty.

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Ms Charles and Mr Dunn travelled to the US last week in an attempt to put pressure on authorities to return Ms Sacoolas to the UK.