The family of teenager Harry Dunn say they believe "there has been a cover up on both sides of the Atlantic" over the investigation into the woman who killed their son.
The comments - made through their spokesman online - come as Foreign Secretary Dominic Raab blamed Northamptonshire Police for a 10-day delay in telling Harry's parents that Anne Sacoolas had left the UK.
The 19-year-old was killed as he rode his motorcycle in Northamptonshire - close to an American military base.
Mrs Sacoolas, the wife of a US intelligence officer, has admitted she was driving on the wrong side of the road having left RAF Croughton and hit Harry. He died from his injuries.
After being questioned by Northamptonshire Police, she left the UK and returned to the US. Officials there have repeatedly said she will not return to Britain to face the justice system.
Since then, Harry's parents, Tim Dunn and Charlotte Cross, have travelled to the US, making a series of television appearances and meeting with President Donald Trump.
As they travelled back today, their spokesman issued a series of tweets.
In them he said the family's search for justice now went beyond fighting to get Mrs Sacoolas back to the UK.
"The family is now concerned that there has been misconduct and a cover up on both sides of the Atlantic and they are intent on exposing it with the help of their lawyers. "Harry's family are being incredibly brave in their darkest hour and are taking it upon themselves to do what ever it takes to ensure that this never happens to anyone ever again. " 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. "Let's see if we can find out why, hold those responsible to account, learn the lessons, and help to ensure that this never happens again."
This week it emerged that the Foreign Office had asked Northamptonshire Police to hold off telling Harry's parents that Mrs Sarcoolas had left.
But today Foreign Secretary Dominic Raab said they had requested a delay of "one or two days" - and said it was the police force's decision to withhold that information for 10 days.
"The police are ultimately responsible for liaising with the family. They take the judgement call. I believe it was 10 days that the delay was for. They [Northamptonshire Police] would have had their reasons for that.
Northamptonshire Police refused to comment on the claims made by Mr Raab.