British detectives are to fly to the United States to interview the suspect in a crash which killed Harry Dunn.
The 19-year-old died in a crash outside RAF Croughton with a car owned by US citizen Anne Sacoolas, who later left the UK claiming diplomatic immunity.
Northamptonshire Police is set to confirm later officers will travel to speak to her as part of its inquiry.
The Dunn family described the news as “perplexing” as they believed the investigation had been completed.
Family spokesman Radd Seiger said a police family liaison officer had told Mr Dunn’s family on Sunday that the police file had been handed over to the Crown Prosecution Service.
“We thought that was good and were waiting for a decision on whether they were going to charge her and then bring extradition proceedings,” he told the BBC.
“So to hear tonight [Monday] from the same liaison officer that Northants Police are now travelling to the United States to interview Mrs Sacoolas is, to put it mildly, confusing and perplexing.”
He said the family would await Tuesday’s planned police press conference to get an accurate picture of what was going on.
“Every passing development is just compounding the family’s misery,” he added.
Shadow foreign secretary Emily Thornberry told BBC Breakfast she had asked for all correspondence between the US embassy, Foreign Office and Northamptonshire Police to be made public.
Ms Thornberry, who is due to meet the Dunn family later, said: “It seems to me an awful lot of decisions have been made by officials at the Foreign Office and in the end these are the sorts of decisions which should be made by politicians.
“Politicians should have been involved at a senior level at an early stage and made clear we were not prepared to put up with this.
“It seems to me that over the last three years our relationship with the United States has been one whereby the British have been tiptoeing around and not wanting to upset the Americans.”
‘Insult to injury’
Ms Thornberry said “ambiguity” over Mrs Sacoolas’s diplomatic status needed to be cleared up but it was a positive sign the Foreign Office had been advised there was “no reason” not to begin extradition proceedings.
On Monday evening, Foreign Secretary Dominic Raab said the US Embassy had informed the British government Mrs Sacoolas would be leaving the UK.
He told the Commons his department asked for her diplomatic immunity to be waived, but the request was refused by the US.
Mr Raab also revealed police waited for 11 days to tell Mr Dunn’s family that Mrs Sacoolas had flown home.
The Dunn family said his statement “added insult to injury” and there was an “unacceptable lack of information being provided to the family”.
Northamptonshire Police has yet to respond to the revelation but Chief Constable Nick Adderley tweeted on Monday the force had at all times “acted with the upmost integrity and transparency”.
Mr Dunn died from his injuries when his motorbike and a car collided outside the RAF station in Northamptonshire on 27 August.
Mrs Sacoolas’ husband was reportedly stationed at the base as an intelligence officer.
At the time of the crash she had diplomatic immunity, but both the British and US governments agree that by returning to the US she had forfeited that right.
Mr Raab said he had commissioned a review into immunity arrangements for US personnel and their families at the RAF Croughton annex in light of this case.
Mr Dunn’s family were due to meet with Mr Adderley on Wednesday but were told he could not say anything more than offering his condolences.
“They feel completely abandoned by both [the police and the Foreign Office],” Mr Seiger said.
“This is incredibly stressful and exhausting and gruelling. The family just want answers.”