Video report by ITV News Correspondent Neil Connery
The family of Harry Dunn has called on Dominic Raab to resign after pictures emerged of the Foreign Secretary at a reception, hosted by the US ambassador, three days before the suspect in the teenager's death left the country.
Charlotte Charles, Harry's mother, told ITV News Mr Raab "needs to step down" as the family "don't have any faith left in him."
They said seeing "drinks flowing, celebrating life" as they prepared for the 19-year-old's funeral was the straw that broke the camel's back.
Harry was riding his motorbike near RAF Croughton in Northamptonshire in August last year when he was in collision with a car driven by Anne Sacoolas, the wife of a US intelligence officer.
Mr Raab attended the event at Winfield House, residence of Ambassador Woody Johnson, on September 12.
Pictures were posted on social media the day after - and Mrs Sacoolas left the UK on September 15, with the US government saying she had diplomatic immunity for any potential prosecution.
Harry's funeral took place on September 18.
Ms Charles told ITV News: "[Mr Raab] needs to step down. He needs to resign, he needs to go, or the prime minister needs to make sure he goes.
"We are absolutely at the point now where we can't take much more with him being in charge at the FCO.
"We've been lied to. It's not just about what we found out this week.
"It's the culmination of absolutely everything. They've met with us on many occasions.
"He's had the opportunity to talk to us, treat us like humans, he hasn't done that."
She added: "It's got to stop. We don't trust him. We haven't trusted him for a while.
"We don't have any faith left in him.
"He is the one who allowed her to go back to the US. We can't trust him or rely on him to be the one to bring her back.
"We can't cope with him being there anymore."
Harry's parents said they felt "sick" after seeing pictures of Mr Raab at the US Ambassador's party reception.
Tim Dunn, Harry's father, spoke of his first reaction to seeing the pictures: "It's really upsetting, sickening. It was just like they're telling us one thing, there's obviously something else going on.
"We've always felt that something else is going on completely to what they're saying and the more and more stuff keeps coming forward... and then when you see stuff when they're having a drink and a laugh.
"They're saying they didn't know until the following 3/4 days later.
"I'm sorry I just can't believe that. They must have talked about it. They must have known. It seems like they're telling us one thing but doing another thing."
ITV News has seen documents revealing some of what happened inside the Foreign Office.
Initially, officials took the view that under the special arrangements covering the RAF Croughton base where her husband worked that an advance waiver to any diplomatic immunity would take effect.
The Foreign Office believed the Croughton arrangements' drafters would not have intended to provide greater immunity for family members than for diplomatic staff.
But by August 30, a US Embassy official said waivers did not apply to family members and the US considered she had diplomatic immunity.
On September 13, the US embassy informed the Foreign Office that Anne Sacoolas and her family would be leaving the UK the next day and that diplomatic immunity would not be waived.
The Foreign Office insists it objected in the strongest possible terms and has continued to do so.
The night before, the US ambassador Woody Johnson hosted his end of Summer reception at his Winfield House residence, attended by, among others, the Foreign Secretary Dominic Raab and International Trade Secretary Liz Truss.
Sources at the Foreign Office say Mr Raab was unaware of the death of Harry Dunn or the involvement of Anne Sacoolas when these pictures were taken.
A FCO spokesperson said as soon as the Foreign Secretary was made aware the US had removed Anne Sacoolas, he raised the case at the highest level with the US government, including with US Secretary of State, Mike Pompeo.
Ms Charles spoke of her reaction to the pictures, she said: “Sick doesn’t even come close...celebrating life, drinks flowing, again it just proves to me that Harry’s life really didn’t matter to them at all.
"It was more evidence for me that Harry was the most unimportant thing to them ever and what I hope they’re starting to learn is that he was the most important thing to us.
"I still believe now they just don’t care.”
A day after that tweet - September 14 - a senior Foreign Office official texted a US embassy counterpart to say: "I think that now the decision has been taken not to waive (immunity), there's not much mileage in us asking you to keep the family here.
"It's obviously not us approving of their departure, but I think you should feel able to put them on the next flight out...”
The FCO told ITV News it “acted properly and lawfully in relation to Harry’s death” and said it has “consistently called for immunity to be waived” but said the police were not able to lawfully prevent Anne Sacoolas from leaving the UK.
They say the withdrawal of the family was a US decision.
The only communication released by the White House in September between President Trump and the Prime Minister is a phone call on September 18th.
On the same day hundreds of friends and family attended Harry Dunn’s funeral at Banbury Crematorium in Oxfordshire.
Six days later, Boris Johnson met Donald Trump in New York during a visit to the UN General Assembly.
A Downing Street spokesman said this week the prime minister was unaware of the Harry Dunn case until after Ms Sacoolas had left the country.
His spokesman said: "My memory is that no he wasn't aware."
The story broke on October 4th. Three days later, Boris Johnson made his first public comments.
He said: "Everyone's sympathies are very much with the family of Harry Dunn and our condolences to them for their tragic loss and no I must answer you directly I do not think that it can be right to use the process of diplomatic immunity for this type of purpose. I hope that Anne Sacoolas will come back and will engage properly with the processes of law as they are carried out in this country."
The FCO said the prime minister was notified about the case after September 15.
The Dunn family want to know if Mr Johnson knew of the events surrounding Mrs Sacoolas heading back to the US.
Ms Charles said: “I’ve no idea who to believe. The more and more we find out, the harder it is to try and absorb it all.
"If he didn’t know, if the prime minister wasn’t aware, who on earth in the Foreign Office made that decision not to tell him?
"I would have thought Dominic Raab would have gone straight to the prime minister for advice.
"Serious questions need to be asked.”
And Mr Dunn, added: “You'd like to think that he [Boris Johnson] was told within the first couple of days that there's an accident, someone had died and this person was about to leave the country under immunity.
"You'd like the think the prime minister would have been on the case with the FCO looking into it to make sure everything was done correctly. What were they thinking not to go upstairs to the boss?
"Surely it's a massive case to get right. You can't just wing it."
A Downing Street spokesman told ITV News: "PM has engaged with this case from the outset and has raised it on a number of occasions with Trump directly. He believes Sacoolas should face justice in the UK."
The Crown Prosecution Service charged Anne Sacoolas with Causing death by dangerous driving in December 2019.
It said US Consular staff dependants outside London did not enjoy diplomatic immunity.
The US Government refuses to extradite her claiming she had diplomatic immunity.
Last week, it emerged she is now a "wanted internationally suspect."
The US State Department told ITV News: "Since the tragic accident occurred, the United States has been closely engaged with the UK government, and we have been transparent about our positions on legal and diplomatic matters concerning this accident."
A FCO spokesperson said: “We have the deepest sympathy for Harry’s family. No family should have to experience what they have gone through. The case remains of the highest priority for the Foreign Secretary who continues to raise the case with the US Government, including last week.
“Both the Foreign Secretary and the Prime Minister have been clear with the US that the refusal to extradite Anne Sacoolas amounts to a denial of justice, and that she should return to the UK.
“We are committed to revising the arrangements at RAF Croughton to ensure they cannot be used in this way again. The Foreign Secretary remains ready to meet Harry’s family and to support them to get the justice they deserve.”
The FCO told ITV News a judicial review is underway into its actions and the police’s actions in relation to Harry’s death and they “are engaging with it in full.”