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.

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