Northamptonshire Police have said they were not informed of the “secret agreements” between the US and the UK that allowed Harry Dunn’s alleged killer to claim diplomatic immunity, according to court documents.

The papers, seen by the PA news agency, show the force also claims that the Foreign Office (FCO) did not tell them about the risk of Anne Sacoolas being withdrawn from the UK.

The Dunn family have brought judicial review proceedings against the Foreign Secretary and Chief Constable Nick Adderley over the immunity claimed by Sacoolas.

They say Northamptonshire Police’s new grounds for defence show that the FCO kept the force “in the dark” during their investigation.

During a court hearing last month, the family’s lawyer Geoffrey Robertson QC said the case “turns on the interpretation of a secret agreement made in 1995… between the US and UK”.

Mr Dunn, 19, was killed when his motorbike crashed into a car outside a US military base in Northamptonshire on August 27 last year.

Sacoolas, 43, claimed diplomatic immunity following the crash and was able to return to her home country, sparking an international controversy.

The family claim Sacoolas, in line with agreements made between the UK and the US, should have had her claim to immunity pre-waived.

The Foreign Office argues that, because Sacoolas was notified to them as a spouse, and dependants were not mentioned in the so-called “Exchange of Notes” agreement, her immunity was not pre-waived, in what they have described as an “anomaly”.

PA has now seen documents in which the second defendant in the judicial review proceedings, Northamptonshire Police, detail eight separate pieces of information the FCO did not tell them about the case before the suspect’s departure.

The force have said they were not sent or informed about the so-called “Exchange of Notes” which allowed Sacoolas to claim immunity, or the existence of a pre-waiver.

Lawyers acting on behalf of Mr Adderley have also said they were not informed of the pre-waiver in place for Sacoolas’s husband Jonathan, preventing them from interviewing him as a witness.

Other key facts the force said they were not informed of include the FCO’s provisional position that Sacoolas did not have immunity, that a decision had been made to tell her family to “feel free” to get on the next plane out of the UK, and that the US had declined a waiver for her immunity.

Reacting to the police’s new grounds for defence, family spokesman Radd Seiger told PA: “The chief constable’s amended case in the Judicial Review makes for truly astonishing reading and sinks the foreign secretary’s frequently repeated public line that he and his team handled the Anne Sacoolas affair ‘lawfully and properly’.

“The chief constable now systematically pulls apart that argument, detailing now as he does, the precise extent to which the FCO kept Northamptonshire Police in the dark.

“They were not told about the lack of diplomatic immunity in Jonathan Sacoolas’s case and the ambiguity in respect of Anne Sacoolas’s claim that she had it, venting his anger in the process that he and his team were obstructed from pursuing justice following Harry Dunn’s tragic death.

“It appears to me that the FCO deprived Northamptonshire Police of the opportunity to interview a key witness to the crash by not informing them that Mr Sacoolas had no diplomatic immunity – it’s astonishing and extremely worrying.”

The FCO maintain they have acted lawfully and properly throughout the investigation.

A spokesman for Northamptonshire Police said: “On June 18, 2020, the High Court granted the parents of Harry Dunn permission to seek a judicial review of the actions of the secretary of state for the Foreign and Commonwealth Office and the chief constable of Northamptonshire Police.

“At the same time, the lawyers acting for the parents of Harry Dunn amended their Detailed Statement of Facts and Grounds.

“The High Court gave the secretary of state and chief constable permission to respond to those amendments.

“We have therefore amended our Detailed Grounds of Response and these have today been served on the parties in accordance with the timetable set by the High Court.”