London (CNN) The diplomatic immunity of the wife of an American diplomat who is the suspect in a fatal traffic collision that killed a British teenager "is no longer pertinent," the UK Foreign Secretary said Sunday.

Sacoolas was originally cooperating in the case, police have said, but she left the country despite assuring them she would remain in the UK.

"The US have now informed us that they too consider that immunity is no longer pertinent," UK Foreign Secretary Dominic Raab said in a letter written to Dunn's family.

"We have pressed strongly for a waiver of immunity, so that justice can be done ... Whilst the US government has steadfastly declined to give that waiver, that is not the end of the matter," Raab said in the letter seen by CNN.

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