In a statement released over the weekend, Northamptonshire Police say that a 42-year-old American suspect - Mrs Sacoolas - "engaged fully" with the force's investigation in the weeks after the crash - and even claimed she had "no plans" to leave the country.
He said the police had "CCTV evidence" that the vehicle had left the air base and was "on the wrong side of the road" when it collided with Mr Dunn.
Intelligence base: The accident happened when Mrs Sacoolas allegedly pulled out of RAF Croughton on the wrong side of the road.
Dunn's mother Charlotte Charles told Sky News that she is "utterly broken", following the accident and that she and her family want justice.
Under the 1961 Vienna Convention, diplomats and their family members are typically immune from prosecution in their host country.
Foreign Secretary Dominic Raab has already raised the case with the USA ambassador to Britain, Woody Johnson.
Harry Dunn, 19, died in a head-on collision while riding his motorbike. The case will receive intense attention at senior levels, the spokesperson said, and the department will consider its global impact, as it does with all cases like this.
The mother, who works in a GP's surgery, said her family were devastated and heartbroken but "totally deprived of the ability to grieve".
But Prime Minister Boris Johnson urged the U.S. Embassy on Monday to re-think its decision to invoke immunity for Sacoolas. "We will go to Washington if we have to", she added. The British government is now petitioning the US government to surrender that immunity, so she can be brought back to the United Kingdom for investigation and trial.
Responding to a question on Twitter on Sunday, Mr Adderley confirmed that he and Stephen Mold, Police Fire and Crime Commissioner for Northamptonshire, had contacted the embassy for immunity to be waived. Dunn's family members and British authorities pleaded that she return to the country to stand trial.
I do not think that it can be right to use the process of diplomatic immunity for this type of goal.
"The next minute we found out that actually, she and her family had left, so it is really disappointing, which prompted the letter from me to say we want to see justice done and the family have a right to see justice done", he said.
On Tuesday Dunn's parents described him as a "big-hearted" person and "passionate" biker who "lived to ride".
The teenager's father Tim Dunn said: "It's appalling".