The Foreign Secretary was “cold” and “rude” to the family of Harry Dunn during a meeting about the teenager’s death, their spokesman has said.
Dominic Raab met the 19-year-old’s parents, Charlotte Charles and Tim Dunn, last week.
The teenager was killed when his motorbike was involved in a head-on collision outside RAF Croughton in Northamptonshire in August.
His parents met Raab after Anne Sacoolas, the motorist allegedly responsible for the crash, was given diplomatic immunity and allowed to flee to the US after the crash.
Family spokesman Radd Seiger told the PA news agency: “They were twitchy (at the meeting with the family). There were three lawyers in the room – why would you bring three lawyers into a room with a grieving mother? Why?
“There were 12 people in a room – for a meeting that was supposed to be him expressing his condolences.
“He (Raab) was stiff, he was cold, he was unpleasant, he was rude. Then three days later we get another letter.
“That isn’t right.”
His comments came as the family demanded an investigation over the Foreign Office’s (FCO) advice to Northamptonshire Police that Sacoolas had diplomatic immunity.
They are demanding to see all emails, messages and notes sent in relation to her immunity status.
Speaking in New York, Seiger said: “We want to conduct an investigation into the FCO’s decision to advise Northamptonshire Police that this lady had the benefit of diplomatic immunity.
“What we don’t know is whether somebody cocked up or whether they were put under pressure by the Americans to concede.”
If they are not happy with the documents, the family say they will then ask for a judge’s opinion on the lawfulness of the FCO’s decision.
Seiger added: “If we’re not satisfied, then we’ll go to a judicial review and ask a High Court judge to review it all.”
On Saturday, before the family left for the US, they received a letter from Raab, saying that Sacoolass no longer had immunity.
“The letter is worded very carefully, they’re not saying it’s just a change of heart – they are saying that it’s an evolution,” said Seiger.
He also said that the family’s lawyers, Mark Stephens and Geoffrey Robertson QC, believe “they’ve (the FCO) been lying to you”.
Charles and Dunn said: “We were astonished last Saturday to receive the hand-delivered letter from the FCO indicating that Sacoolas did not in fact have diplomatic immunity.
“We were still reeling from our terrible meeting with Dominic Raab, Foreign Secretary, last Wednesday, just three days earlier, where he was adamant that Mrs Sacoolas did have immunity.
“Something is not right.
“We will not rest until we get the truth. We will not let Harry passing be in vain.”
Sacoolas has said she is “terribly sorry” about the incident and that she had “no time to react” when she saw Dunn’s motorbike.
Her lawyer, Amy Jeffress, told the Mirror: “Anne was driving on the wrong side of the road and is terribly, terribly sorry for that tragic mistake.
“She wants to meet the family and apologise and take responsibility. Anne had no time to react when she saw the motorbike.”
Jeffress, from US firm Arnold & Porter, said the diplomat’s wife also stayed at the scene and flagged down other people to help the teenager.
“She spoke to Harry to tell him that she would call for help.
“She waved down another car. That driver offered to assist Harry so that Anne could comfort her young children in her car.”
The FCO has been contacted for comment.
