Sacoolas left the UK on 15 September 2019, but in her report, Dame Anne said the family were not officially told this – as well as what her name was and that she had immunity – until 11 days later on 26 September.

Mr Dunn’s father was already aware Sacoolas’s children had left the country because he was head of maintenance at their school, the report said.

“This understandably created distrust both of the message and the messengers,” she said.

“The belief that there had been a conspiracy between the UK and US authorities to secretly ‘spirit her out’, with information deliberately withheld from the family.”

In a statement, Mr Dunn’s mother Charlotte Charles said the review had been “a hugely emotional experience” that “triggered a lot of anger”.

“Having turned to the authorities for help, we got nothing from them,” she said.

Mr Dunn’s father Tim Dunn said: “The hardest part is knowing that more could and should have been done for our boy in those early days.”

The report, published earlier, examined actions taken by the department – now called the Foreign, Commonwealth and Development Office (FCDO) – between the teenager’s death on 27 August and the end of December 2019.

A police inquiry concluded that Sacoolas was not arrested at the scene because she was deemed to be in a state of shock and it was not deemed necessary at the time.

Dame Anne’s report explained that she was a US State Department employee, but had diplomatic immunity at the time because she was the wife of a US intelligence officer.

The then-foreign secretary was not told the family had left the UK until the following day, said Dame Anne.

After Sacoolas’s departure, Dame Anne said the FCDO asked Northamptonshire Police for a “day or two” to “get our ducks in a row” before informing family.

The department then asked police not to mention its request – regarding the delay – when the force met the family on 26 September.

Sacoolas pleaded guilty to causing death by careless driving in 2022 and was given an eight-month suspended jail term.

Dame Anne recognised that the Protocol Directorate – the FCDO department responsible – was under “huge pressure”.

She made a series of recommendations for the FCDO, including:

• An “immediate surge of resources” with “early ministerial involvement” when responding to exceptional circumstances
• A strategy to communicate and engage with families and victims
• The skills and experience of consular staff should be deployed when needed in sensitive cases involving diplomats in the UK

“The issue was not recognised as a crisis and escalated to a sufficiently high level at an early stage, losing opportunities to influence, rather than respond to, events,” said Dame Anne.

“Direct communication with the family was late, sporadic and often overtaken by events, and the FCDO was slow to recognise that the family were allies in achieving justice and securing other necessary changes.”

The Conservatives were in office at the time of Mr Dunn’s death.

The loophole, which meant Sacoolas had diplomatic immunity, was closed in 2020.