London (CNN)The United Kingdom has urged the return of an American diplomat's spouse who left the country while a suspect in a fatal traffic collision.
In a written statement on Friday, Foreign Secretary Dominic Raab said he had called the US ambassador over the case and expressed the UK's "disappointment."
Northamptonshire Police Superintendent Sarah Johnson said in a statement on Saturday that a 42-year-old American woman "being treated as a suspect in our investigation into a fatal road traffic collision ... has left the country."
Harry Dunn, 19, died in the accident while riding his motorbike, according to a statement from his family. The accident took place on August 27 in Croughton, Northamptonshire, home to a Royal Air Force station controlled by the US Air Force.
Johnson said the woman previously had been cooperating fully. She had told the force she did not intend to leave the country in the near future.
Northamptonshire police added they were "working closely with the Foreign & Commonwealth Office" and "exploring all opportunities through diplomatic channels to ensure that the investigation continues to progress."
Speaking to Sky News, Johnson said: "We were aware that diplomatic immunity had been raised as an issue, and we then made an immediate application for a waiver in order for us to undertake further investigation and interview. Unfortunately, we were latterly advised that the waiver had been declined and that the suspect had left the UK."
'Deprived of the ability to grieve'
The UK Foreign Office would not comment on whether the diplomatic immunity of the suspect had been raised with the British authorities.
In a statement on Facebook, the victim's mother, Charlotte Charles, said: "With the terrible news that the other driver has left the country, seemingly in an attempt to evade justice, we are totally deprived of the ability to grieve and our lives are now painfully on hold." She added that the suspect "must return to the UK to face justice."
CNN has tried to contact the US Embassy in London for comment but could not immediately reach officials. CNN also has contacted the State Department for comment.
Johnson said Dunn's family "deserve justice and in order to achieve this, a full and thorough investigation, with the assistance of all parties involved, needs to take place."
Cabinet minister Andrea Leadsom, who is a member of Parliament for South Northamptonshire, called for "proper justice" for Dunn on Twitter on Saturday.
"I met Harry's family yesterday. They are totally heartbroken. We have to get proper justice for Harry and closure for his family," she wrote.