The family of a teenager who was killed in a car crash with the wife of an American diplomat have said they are planning to sue the Trump administration for lawless misconduct and a cover-up.

Harry Dunn‘s parents will also launch a civil case against Anne Sacoolas, whose lawyers have admitted she was driving on the wrong side of the road before her car hit 19-year-old’s motorcycle on 27 August near RAF Croughton in Northamptonshire.

After the 42-year-old was questioned by police following the crash she returned to the US under the protection of diplomatic immunity and has refused to return to the UK.

Mr Dunn’s parents met with Donald Trump earlier this month at the White House, where the US president described the death as a “terrible accident”, but made it clear the US would not extradite Ms Sacoolas.

Radd Seiger, a spokesman for the Dunn family, said they have found lawyers willing to work with them to pursue a “civil claim for damages” against Ms Sacoolas, as well as a claim against the Trump White House.

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“The family did not want any of this”, Mr Seiger said, “They simply wanted the police and authorities on both sides of the Atlantic to do the right thing for them and ensure justice was done.”

He added “We are bringing claims against both Ms Sacoolas in the USA for civil damages as well as the Trump administration for their lawless misconduct and attempt to cover that up.

“No one is above the law and the family are determined to ensure that this never happens to another family again. It will be Harry’s memorial. His legacy.”

He went on to accuse the Trump administration of trying “to twist and contort the laws on diplomatic immunity to argue that Ms Sacoolas should be permitted to skip the UK after her actions and escape justice”.

He said: “That is not how it works and the family will not stand for it. They intend to take their case to federal court and expose the Trump administration’s actions here for what they are, a wanton and total disregard for the law.”

Earlier this month, the family also made it clear that they would be seeking legal action against the Foreign Office for their handling of the incident.

They have also referred Northampton police to the Independent Office for Police Conduct.

Mr Seiger’s announcement came shortly after a former senior ranking Metropolitan Police officer labels the investigation into the death of Mr Dunn as “very, very poor”.

Dal Babu, who was a chief superintendent until his retirement in 2013, said Northamptonshire Police had shown a “lack of compassion” towards the teenager’s family and he had “a lot of concerns” about parts of the investigation.

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“There seems to be a lack of compassion from Northamptonshire Police, the Foreign Office and the government”, he said. “There was confusion over diplomatic immunity and whether the suspect had it or not. All in all, the investigation seems to have been very, very poor in terms of family support.”

Questioning the methods used by police during the investigation, Mr Babu added: “In terms of the investigation, there are a few things to be factored in – like seizing her passport and whether a test was done for alcohol or drugs.