Harry Dunn's parents have claimed they have been repeatedly "lied to" by officials but will continue fighting for their son.
Charlotte Charles and Tim Dunn said they wanted "justice on UK soil" over the 19-year-old's death, who died in a car crash outside RAF Croughton in Northamptonshire on August 27.
Anne Sacoolas, the wife of a US diplomat who was driving the other vehicle involved, was questioned by officers at the scene and promised to stay in the country while cooperating with the investigation.
But she left the UK and has not returned for further questioning. The Foreign Office has now confirmed Ms Sacoolas no longer has diplomatic immunity.
Speaking today on ITV's This Morning, the couple say they want to find out how Ms Sacoolas was able to return to America after promising to stay in the country , telling presenters Eamonn Holmes and Ruth Langford: "We've been lied to".
Asked by Mr Holmes whether they trusted what the UK authorities, including Secretary of State Dominic Raab, they said they were "sceptical".
Ms Charles said: "We thought it would be straight forward. We had no idea it would become this scale. If justice is going to happen it needs to be on UK soil."
The couple were speaking in their first live interview since meeting President Donald Trump in the US.
The parents had travelled there initially to tell their story to the American public but were called to the White House for a meeting with Mr Trump.
Appearing on the show this morning, they spoke about how they refused to meet with Ms Sacoolas, who was in a room next door during their impromptu meeting in the Oval Office.
They claimed President Trump's national security advisor Robert O'Brien told them in an "aggressive" way that Ms Sacoolas would never return to the UK.
Mr Dunn's parents, who each wore a green ribbon being made by locals in their home village of Charlton, Oxfordshire, as a symbol of their campaign, said they would continue to fight for their son, but admitted their hopes of forcing Ms Sacoolas back to the UK were dwindling.
Ms Charles said: "Her two children were in the car and they must be extremely traumatised and I don't know how they'll move on with their lives if she doesn't do the right thing and come back.
"She killed our son and she has to come back for that.
"We wanted to work with the police to get her a suspended sentence to not take her away from her own children."
Eammon Holmes asked the parents if they felt they would "win against politics", saying: "This is not about justice or rule of law. Does it feel like politics to you?"
The parents agreed politics was playing a part in the negotiations but said it would not stop them fighting for "the truth" about what happened following their son's death.
"It's that simple, we've raised our children to be honest, and if you make a promise you don't break it," said Ms Charles.
"The promise we made to him is giving us the fire to keep going."
Directing her words to Ms Sacoolas, she added: "Just get on a plane and come back to the UK. We're not horrible people."