ITV News has seen some of the confidential documents used by the Foreign Office to assess whether Anne Sacoolas had diplomatic immunity in the days following Harry Dunn's death.
The papers concern secret arrangements covering RAF Croughton where her husband worked as a US military intelligence officer.
The agreement was covered by an Exchange of Letters between the US and UK in 1995 and 2001. The documents we've seen include diplomatic notes and sensitive discussions between the two governments.
They make clear that when the arrangements were drawn up both governments agreed that US staff covered by diplomatic immunity at RAF Croughton would face an automatic waiver from that immunity if they were to commit a criminal act outside their duties.
The key issue in the Harry Dunn case is whether Mrs Sacoolas - as a spouse - had diplomatic immunity at the time of the crash. Nothing in the agreement, or in the discussions at the time it was drawn up seen by ITV News, makes any reference to whether staff spouses have immunity.
But the Dunn family's legal team say she didn't have diplomatic immunity. Even if she did, they say, it should have been waived.
A 1995 draft of the 'Croughton' agreement we've seen outlines the UK government's position on the base.
It says the government is “only willing to accept ...persons as members of the A&T (Administrative and Technical) staff of the US embassy in London with the privileges and immunity ... on the understanding that the US Government ... waives the immunity from criminal jurisdiction of these employees in respect of acts performed outside the course of their duties."
In August 2000, a Foreign Office letter to the US embassy about the arrangements says: "You would also agree to a waiver of immunity from criminal jurisdiction."
A diplomatic note from the US embassy in October 2000 refers to a letter from the US Ambassador William J. Crowe, Jr. in August 1995. It says he accepted the offer to "grant diplomatic agent status ...on the condition that the US Government agrees in advance to waive the immunity from criminal jurisdiction in respect to acts performed outside their course of duties."
Documents we've seen say, two days after Harry Dunn's death, the Foreign Office's initial view was that the advance waiver in the RAF Croughton agreement might apply and there was no need for any further waiver of immunity.
A Foreign Office official stressed this advance waiver from immunity to the US embassy.
But the next day, a senior US embassy official told the Foreign Office that advance waivers at the base only applied to members of administrative and technical staff, and did not extend to their family members. The US said Mrs Sacoolas had diplomatic immunity.
The Foreign Office eventually accepted the US view that the Sacoolas family had immunity. But the Foreign Office insists it objected in the strongest possible terms and has consistently called for Mrs Sacoolas's immunity to be waived and that she should return to the UK to face justice.
The Foreign Office argues that no mention of spouses is made in the RAF Croughton agreement. It says under Article 32 of the Vienna Convention on Diplomatic Relations any waiver of immunity from criminal prosecution must be made clear. It argues the absence of any reference to automatic waivers for spouses means Mrs Sacoolas had diplomatic immunity.
Harry Dunn’s mother, Charlotte Charles, told ITV News: “Obviously, most of this goes over our heads and we are leaving this up to our advisers and lawyers.
"But our feelings all along have been that no one is above the law. Our Harry died and the person responsible was allowed to leave in an attempt to evade justice.
"I am relieved that this is all now coming out in the open, so that we can get justice for our son and help to ensure that this never happens to another family again.”
When the Crown Prosecution Service charged Mrs Sacoolas in December 2019 - after more than three months consideration by its independent lawyers - it said there was no diplomatic immunity for dependants of consular staff outside London. If there had been it would have been an impediment to them charging here with causing death by dangerous driving.
The Dunn family's representative Radd Seiger said: "The arrangement at RAF Croughton was kept deliberately secret from Parliament when they were drafted up. They have never been scrutinised before nor tested in Court.
"As we have all seen with the Anne Sacoolas affair, the arrangements are far from satisfactory. We are confident that the Courts will find that no one at Croughton has immunity. It will then be for Parliament to consider who if anyone should benefit from these privileges going forward.“
Labour’s shadow Foreign Secretary Lisa Nandy told ITV News: “Nine months on from this tragic incident, Harry’s family should not be in the dark, fighting for clarity about the crucial issue of whether diplomatic immunity applied in this case.
"This underlines the need for the Foreign Secretary to make an urgent statement to Parliament clarifying the many inconsistencies in this case.
"Harry’s family deserve to know that mistakes will be rectified and no family in the future will have to go through a similar ordeal.”
The Foreign Office said: “We consistently called for Anne Sacoolas’s immunity to be waived before she left the UK. Both the Prime Minister and the Foreign Secretary have been clear with the US that the refusal to extradite her amounts to a denial of justice, and that she should return to the UK.
“We have the deepest sympathy for Harry’s family. We have done and will continue to do everything we properly can to ensure that justice is done.”
A judicial review hearing on the case brought by Harry Dunn's family against The Foreign Office and Northamptonshire Police is due to be held next month.