The chief constable of Northamptonshire Police has urged the suspect in the death of a Northampton Town fan to come back to England and face justice.
Speaking on Radio 4 on Monday morning, Chief Constable Nick Adderley said the wife of a US diplomat should "do the right thing and come back".
Harry Dunn, 19 of Charlton near Brackley, died in hospital earlier this year, following the collision on the B4031 Park End, Croughton. He was riding a motorcycle at the time of the head-on collision with a car.
His family paid tribute to him, describing him as a "caring and loving young man".
Over the weekend it emerged that a suspect in the case had fled the country, claiming diplomatic immunity, and had returned to America.
The force is working with the UK government to negotiate her return but investigators know that their options are limited due to the laws around diplomatic immunity.
The chief constable appealed to the suspect directly to waive her immunity and face justice.
"I am father, I know how I would feel if it was my son that was killed, She's a mother and I am sure she will know how she would feel if it was her own child that had come to harm. I would say, come back face justice, let's see what happens. Do the right thing both morally and come back," he said.
When asked about the "special relationship" between the US and the UK, the chief constable said: "I don't want to castigate the whole American community we have in Northamptonshire and we have a very good relationship with that community and that will continue.
"This is a tragic incident that has happened in Northamptonshire - we hope that special relationship is pushed up and we get the lady back to face justice."
Mr Adderley said there had been precedent set in the past when diplomatic immunity had been waived by the individual rather than the government.
"We live in hope but the reality is that if they don't want to apply that waiver there is nothing we can do about it," he added.
In a statement issued late over the weekend, Detective Superintendent Sarah Johnson, confirmed that a 42-year-old American woman was being treated as a suspect in the investigation and that she had now left the country.
“Northamptonshire Police followed all of its usual procedures following the incident, including liaising closely with the suspect, who engaged fully with us at the time and had previously confirmed to us that she had no plans to leave the country in the near future," Det Supt Johnson said.
“Due process was also followed in seeking the necessary documentation to allow for the arrest and formal interview of the suspect, and the Force is now exploring all opportunities through diplomatic channels to ensure that the investigation continues to progress.
“We are also working closely with the Foreign & Commonwealth Office in an effort to come to a resolution regarding this matter," she added.
“Harry Dunn’s family deserve justice and in order to achieve this, a full and thorough investigation, with the assistance of all parties involved, needs to take place.
“Northamptonshire Police is committed to ensuring justice for Harry and specially trained officers continue to support the Dunn family in their loss, including keeping them fully informed of all developments in the investigation.”
Speaking to Sky News on Friday night, the parents of Harry called for President Trump to intervene and said they were "a family in ruins".
In a statement issued by the family in August, they said: "Our Harry loved life; his 19 years were always busy. Growing up as a twin and having three other brothers and a sister there was always competition that contributed towards Harry’s bubbly and outgoing nature.
“From the age of seven he shared a passion for riding motorbikes with his Gramps and step-dad. Harry was a free spirit who spent all his wages on fuel for his bike and loved to explore the countryside, with Portland Bill being one of his favourite places.
“As an avid football fan he followed in his Dad’s footsteps and held a season ticket at NTFC for a number of years.
“His love for his family and friends outshone everything and made him the caring and loving young man he was. It’s not until now, with all the messages we have received, that we have come to realise how many people’s lives he has touched.
“The loss of Harry is going to leave the biggest hole in our lives and he will truly be missed by all.”