Harry Dunn’s alleged killer was “employed by an intelligence agency in the US” at the time fatal road crash – which was “especially a factor” in her departure from the UK, a court has heard.

The Alexandria district court in Virginia was told Anne Sacoolas and her husband Jonathan Sacoolas worked for the US State Department and they “fled” the country due to “issues of security”.

The revelations came out during Sacoolas’s application to dismiss a civil claim for damages against her made by the Dunn family on Wednesday.

The suspect’s barrister, John McGavin, told the court he could not “completely candidly” explain why the Sacoolas family left the UK – adding: “I know the answer, but I cannot disclose it.”

Family spokesman Radd Seiger told the PA news agency UK authorities must “urgently investigate whether she had diplomatic immunity” at the time of the incident.

Mr Dunn, 19, was killed after a car crashed into his motorbike outside US military base RAF Croughton in Northamptonshire in August 2019.

Sacoolas, 43, had diplomatic immunity asserted on her behalf by the US Government and she was able to return home around three weeks later.

She was charged with causing the teenager’s death by dangerous driving, but a Home Office extradition request was rejected by the US State Department in January last year.

Both the Donald Trump administration and new President Joe Biden’s administration have described that decision as “final”.

On Wednesday, the court heard that one of the reasons Sacoolas had not returned to the UK was a “fear” that because of the “media attention, she would not have a fair trial”.

Mr McGavin said the suspect was “currently apologetic” and has “accepted responsibility for the accident”.

The Dunn family’s barrister in the US, Agniezska Fryzsman, told the court the British Government had written a letter to the court to “endorse” their claim.

The admission from Sacoolas’s own barrister about her employment at the time of the crash has raised the question of the diplomatic immunity asserted on her behalf.

Under the agreements at RAF Croughton dating back to 1995, anyone working at the base from the US as part of the “administrative and technical staff” would have their immunity pre-waived, meaning they would not be immune from criminal jurisdiction.

In a short statement issued after the hearing, Mr Seiger told PA: “Given the admission in open court by Mrs Sacoolas’s counsel that she was employed by US intelligence services at the time of the crash, the UK authorities must now urgently reinvestigate whether she had diplomatic immunity.

“They have to investigate given that employees had their immunity pre-waived under the 1995 RAF Croughton legal agreement.”

The case was adjourned until a further hearing at the same court on February 17.