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A former senior Met Police officer has slammed the investigation into Harry Dunn's death as "very, very poor".

Mr Dunn, 19, was killed when his motorbike was hit by Anne Sacoolas, the 42-year-old wife of a US diplomat, who is believed to have been driving on the wrong side of the road outside RAF Croughton in Northamptonshire on August 27.

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.

Mr Babu said that he had "a lot of concerns" about parts of the investigation, which he called "very very poor".

It comes as the family's spokesman Radd Seiger announced on Tuesday that they were working with lawyers to pursue a "civil claim for damages" against the suspect Mrs Sacoolas.

Mr Seiger also said that lawyers would be looking into what they called the "lawless misconduct" of US President Donald Trump's administration "and then the attempt to cover that up".

Mr Babu, who served with the Metropolitan Police for 30 years, questioned whether a small force such as Northamptonshire Police had the "necessary expertise" to lead such a complex investigation.

Speaking about the police's involvement in the case, Mr Babu said: "Just looking at it as an outsider, I have a lot of concerns about the investigation and the way the family has been treated.

"There seems to be a lack of compassion from Northamptonshire Police, the Foreign Office and the Government.

"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."

Addressing the role of the family liaison officer (FLO) in the case, Mr Babu continued: "Normally you would have a family liaison officer and from there that individual would be keeping the family updated on what is happening.

"The failure to keep the family informed in this case would be a failure in policing terms - the FLO has to build a relationship with the family.

"The police investigation lacked clarity and transparency for the family and the failure of the family liaison officer to keep the family informed was very disappointing."

Mr Babu also described the police's potential trip to the United States to question Mrs Sacoolas as a "complete waste of taxpayers' money".

He said: "The suspect leaves the country and Northamptonshire Police are now thinking of travelling to the US. That serves no purpose whatsoever.

"It's just a complete waste of public money. She needs to be coming here to face the consequences of her actions."

It comes as the family's spokesman Radd Seiger announced on Tuesday that they were working with lawyers to pursue a "civil claim for damages" against the suspect Mrs Sacoolas.

Mr Seiger also said that lawyers would be looking into what they called the "lawless misconduct" of US President Donald Trump's administration "and then the attempt to cover that up".

He said: "The family did not want any of this. 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.

"However, that is not what happened and it is now clear that the various agencies involved not only did not do what they were supposed to do, but have in fact become part of the problem. There appears to be widespread misconduct and then an attempt to cover that up.

"We are accordingly announcing today that following on from the announcement last week of beginning legal proceedings in London against the Foreign Office, we are announcing today that we are bringing claims against both Mrs Sacoolas in the USA for civil damages as well as the Trump administration for their lawless misconduct and attempt to cover that up."

He added: "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."

Addressing the action the family would take against Mr Trump and his government, Mr Seiger said: "The Trump administration have tried to twist and contort the laws on diplomatic immunity to argue that Mrs Sacoolas should be permitted to skip the UK after her actions and escape justice.

"That is not how it works and the family will not stand for it."

"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."

Addressing the action the family would take against Mr Trump and his government, Mr Seiger said: "The Trump administration have tried to twist and contort the laws on diplomatic immunity to argue that Mrs Sacoolas should be permitted to skip the UK after her actions and escape justice.

"That is not how it works and the family will not stand for it."