The US suspect who fled the UK after teenager Harry Dunn was killed in a crash outside an RAF base has asked to be interviewed by British police under caution.

Anne Sacoolas, the wife of a US diplomat, is believed to have been driving on the wrong side of the road when she hit Mr Dunn’s motorbike outside RAF Croughton in Northamptonshire on August 27.

Shadow foreign secretary Emily Thornberry said she “smelt a rat” over the way the case was handled, and said she would be “digging” on behalf of the family after they visited her on Tuesday.

The spokesman for Mr Dunn’s family, Radd Seiger, said they were in tears as it finally felt like somebody was listening to them following the meeting.

After the crash, Mrs Sacoolas fled to the US, claiming diplomatic immunity, which has since been disputed by the lawyers representing the 19-year-old’s family.

Harry’s parents, Charlotte Charles and Tim Dunn, have made repeated pleas for Mrs Sacoolas to return to the UK to “face justice”, while Mr Seiger has accused the authorities of “further compounding the family’s misery” through “contradictory” actions.

At a press conference on Tuesday, Northamptonshire Police’s chief constable Nick Adderley said the police’s work during the investigation into Mr Dunn’s death was “amongst the best I have ever seen”.

He said: “I will say, as the Chief Constable of the force, having worked in four forces previous to this, I can say that the quality and the standard of investigation that has taken place in this particular case is exemplary – is amongst the best that I have ever seen.”

After the press conference Mrs Charles and Mr Dunn, along with their partners Bruce Charles and Tracey Dunn, met with Ms Thornberry – who on Monday call for all correspondence between Northamptonshire Police, Foreign Office and the US Embassy to be made public.

Speaking after the meeting, the shadow foreign secretary said: “My worry is that over the last three years, we (the UK Government) have been pulling our punches with the current (American) administration, it’s as though we’re worried and scared of upsetting Donald Trump. I just think that’s the wrong approach.

“There are times when you just have to stand up for British citizens. For Heaven’s sake, this family have just lost their teenage boy – if we are not going to stand up for parents like this, what are we about these days?”

Harry Dunn death
Floral tributes on the B4031 outside RAF Croughton, in Northamptonshire, where Harry Dunn, 19, died when his motorbike was involved in a head-on collision in August (Steve Parsons/PA)

She accused the Foreign Office of “running around like headless chickens” in the aftermath of the fatal crash.

Ms Thornberry added: “It’s like bypassing your humanity, it’s not thinking about the grieving family who have lost a teenage boy.

“It’s not putting them at the forefront of your mind. It’s putting something else, and it’s not good – that’s not how it should be.”

After revealing Mrs Sacoolas wanted to be questioned, Mr Adderley urged Mr Seiger, who is acting as spokesman for the Dunn family, to “exercise constraint”.

Asked if he accepted the force had caused unnecessary trauma to Mr Dunn’s family by delaying telling them that Mrs Sacoolas had left the country, the chief constable said: “It was very much a judgment call.

“It’s a call the officers investigating the case have to make and, when we got the information on the day the waiver had been declined, so the diplomatic immunity was now applicable, it was at that time we were informed of that decision.

“It is a decision I actually support. Is it ideal that they found out through different means? No, it’s not ideal.

“But, actually, the decision to tell a grieving family is something we will reflect on and 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.”

Mr Seiger said, despite the family feeling better, some of what Mr Adderley had to say was “inaccurate”.