The woman accused of killing Harry Dunn was working for a US intelligence agency at the time of the crash, a court has heard.
Mr Dunn, 19, died when his motorbike was in a crash with a car near RAF Croughton in Northamptonshire in 2019.
Suspect Anne Sacoolas returned to the US, claiming diplomatic immunity, and an extradition request was blocked.
A court in Virginia has heard her work was “especially a factor” in her leaving the UK.
Her barrister told the court she fled the UK for “security issues” and feared she would “not get a fair trial” if she returned.
The comments about her role were made at a court in the state, where an application to dismiss a civil claim for damages submitted by Mr Dunn’s family is being heard.
When asked by the judge why Mrs Sacoolas had “fled” the UK, her barrister John McGavin said he could not respond “completely candidly”.
“I know the answer but I cannot disclose it,” he said.
Speaking after the hearing, family spokesman Radd Seiger called on the UK government to “urgently reinvestigate” whether Mrs Sacoolas had diplomatic immunity, 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”.
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.
The Dunn’s barrister Agnieszka Fryszman had earlier told the court the British government had endorsed the civil claim and called on the judge not to dismiss it.
Mrs Sacoolas, whose husband was based at RAF Croughton, was charged with causing death by dangerous driving after the crash.
A Home Office extradition request was refused by Donald Trump’s secretary of state Mike Pompeo in January 2020 and last week the new Biden administration said the decision was final.
The judge said he would decide on the application ahead of a further hearing at the same court on 17 February.