Harry Dunn’s parents say they expect UK police to charge a US diplomat’s wife in connection with his death.
Mr Dunn, 19, died after a collision with a car owned by Anne Sacoolas, who was allegedly driving on the wrong side of the road.
His parents Charlotte Charles and Tim Dunn travelled to the US as part of their campaign for justice and met President Donald Trump.
Mrs Sacoolas, 42, went back to the US after the crash in Northamptonshire.
Mr Dunn’s family is due back in the UK later, after their trip to the US to seek justice, following the crash outside RAF Croughton – where Mrs Sacoolas’ husband is reportedly stationed as an intelligence officer – on 27 August.
At the time, Mrs Sacoolas had diplomatic immunity, but both the British and US governments agree that by returning to the US she had forfeited that right.
Radd Seiger, the family’s spokesman, said they have concerns of “misconduct and a cover up on both sides of the Atlantic”.
A statement from the family said: “It is clear that the Americans are desperate to protect Mrs Sacoolas and are intent on ruthlessly and aggressively not letting her return. We are trying to find out why that is. We will not let up in our search for Justice for Harry.
“We now expect Northants Police to take over from the work we have done and the progress we have made, charge her and begin extradition proceedings to bring her back.”
In an interview with ITV, Foreign Secretary Dominic Raab said: “We have done everything we can properly and within the law to clear a path so that justice can be done for the family. And we continue to do so.”
Mrs Sacoolas can only be extradited if she is charged by the Crown Prosecution Service (CPS) with a criminal offence that is serious enough to warrant it.
Northants Police confirmed they were continuing to prepare evidence to hand over to the CPS.
Mr Dunn’s parents rejected a “bombshell” offer from Donald Trump to meet Anne Sacoolas at the White House on Tuesday.
Charlotte Charles and Tim Dunn had felt “a little ambushed” when the president revealed she was in the next room.
Mr Trump described his meeting with the couple as “beautiful” but “very sad”.
Ms Charles and Mr Dunn are due to meet Northamptonshire’s Chief Constable Nick Adderley next week.
Mr Seiger said: “In all my years of practice, I have never seen a family so badly let down after a tragedy and abandoned completely by the system. “
The US State Department has been contacted for comment.