The parents of Harry Dunn have accused the British and American authorities of treating them like "the dirt on the bottom of (their) shoes".
Tim Dunn and Charlotte Charles, whose son Harry died in August, said condolences from the governments were "nothing but empty words" amid their three-month-long saga which has taken them to the White House
Anne Sacoolas, the wife of an American intelligence officer, is believed to have been driving on the wrong side of the road when she hit 19-year-old Mr Dunn's motorbike outside RAF Croughton on August 27.
Mrs Sacoolas claimed diplomatic immunity after the incident and returned to the US, sparking an international controversy.
In an open letter to the UK and US authorities, Crown Prosecution Service and Mrs Sacoolas, Mr Dunn and Ms Charles said: "You have treated us like we are dirt on the bottom of your shoes and we simply do not understand why.
"Anne Sacoolas should never have been allowed to leave and you robbed us of our right to seek justice."
A legal claim against the FCO was issued by Harry's family on October 25 and alleged the granting of diplomatic immunity to Mrs Sacoolas was "wrong in law".
The FCO did not accept the claims.
The family also directly addressed Mrs Sacoolas in the letter, and said: "We wish you no ill will, but you took our son's life that night and you must return to the UK to face the consequences of your actions as anyone else would have to do."
Earlier this month, Northamptonshire Police - who were not told Mrs Sacoolas had left the UK - handed a file of evidence to the CPS after conducting an interview with the 42-year-old in the US.
A decision on charges is yet to be made.
