British and American officials have begun talks to explore the possible extradition of the suspect in the death of Harry Dunn, the Guardian has learned.

The suspect, Anne Sacoolas, went to the US while under investigation after a car she was driving struck a motorcycle ridden by the 19-year-old outside RAF Croughton in Northamptonshire in August.

British police travelled to the US last week and interviewed Sacoolas.

The final part of the evidence detectives from Northamptonshire police were compiling was the transcripts of the interview with Sacoolas.

They have now submitted evidence to the Crown Prosecution Service (CPS), which is expected soon to make a decision about charging the suspect.

One charge under consideration is death by dangerous driving and Dunn’s family want Sacoolas to face justice.

The discussions about extradition so far have been informal, a source said, and exploratory.

Lawyers who handle extradition at the CPS are understood to be aware of the case. But formal proceedings can only start once the CPS, who is the prosecution service in England and Wales, decides to bring a criminal charge which is punishable in both Britain and the US by a sentence of more than 12 months imprisonment.

Sacoolas, who is married to a US intelligence official who was based at the RAF base, claimed diplomatic immunity. The British government said that she no longer had diplomatic immunity. Dunn’s family believe she may never have had immunity.

Mark Stephens, solicitor for the Dunn family, said: “We don’t think she has ever had diplomatic immunity and there is no bar to extradition. It’s the morally right thing and legally right thing to extradite her.”

Police have been investigating the cause of the accident and whether Sacoolas was driving on the wrong side of the road. She cooperated with police and was breathalysed at the scene but left the UK three weeks after the accident aboard a US air force plane.

Dunn’s parents were not told that the suspect in their son’s death had left the UK for over a week.

Stephens said Sacoolas returning to the UK to face any charges would help ease the pain of Dunn’s family: “It would be a way for everybody to get closure. The family want to get closure but in everything they do they are being frustrated.”

Last week in an interview with LBC Radio, US president Donald Trump said he would study the facts of the case but refused to promise that Sacoolas would return to the UK.