The family of 19-year-old Harry Dunn were told there was a “less than 1% chance” somebody would be held accountable for his death.
The completed file of evidence was handed to the Crown Prosecution Service (CPS) on Friday, a month on from a meeting with a senior officer who told them a prosecution was unlikely.
Northamptonshire Police confirmed to the PA news agency that they had completed the file after conducting an interview with the 42-year-old suspect in the case, Anne Sacoolas, in the US.
Mrs Sacoolas is believed to have been driving on the wrong side of the road when she hit the teenager’s motorbike outside RAF Croughton in Northamptonshire on August 27.
She claimed diplomatic immunity after the incident and returned to the US, sparking an international controversy.
Reacting to the file being handed over, the family’s spokesman Radd Seiger told PA: “We are pleased that at long last Northamptonshire Police have been able to complete their investigation and hand the file over to the CPS.
“On September 26 the family were told they had less than 1% chance of somebody being held accountable for the loss of their son.
“The senior officer started the meeting with ‘I have bad news, she has gone and I’m afraid that that means there is less than 1% of somebody being held accountable for his death’.
“It was scary for the family. There was silence and there were some gasps in the room but over the preceding week we had heard rumours that she had gone and that she had diplomatic immunity.”
Mr Seiger continued: “The prevailing emotion was shock – there wasn’t any ranting or raving – there was just shock and anger.
“She then told the family that the investigation would not stop and that they would continue to investigate until they were unable to investigate any more.
“I believe we would not be in this scenario were it not for the incredible determination of the most courageous and dignified family I have ever met – who have moved heaven and earth to get justice for their son on both sides of the Atlantic.”
The lawyers instructing Harry’s family have already announced four separate legal actions after the teenager’s death – against the Foreign and Commonwealth Office, Northamptonshire Police, US President Donald Trump’s administration and the suspect herself.
Speaking about how high the family’s hopes were of legal proceedings in the UK, Mr Seiger told PA: “We have spoken about this at length and it is no longer a matter for us.
“We were simply told a prosecution was not going to happen and we have shaken the tree because we would not accept that.
“We have no interest in calling for any outcome – we are happy to leave it to the CPS and the courts to do their job.”