The alleged killer of teenage motorcyclist Harry Dunn is "wanted internationally" after an Interpol Red Notice was issued for her arrest.
Anne Sacoolas, 42, was charged with causing death by dangerous driving in December but an extradition request submitted by the Home Office was rejected by US secretary of state Mike Pompeo in January.
In an email to the 19-year-old's parents from Northamptonshire Police, seen by the PA news agency, the force said "the wanted circulations should be enacted" should Sacoolas leave the US.
The teenager was killed when his motorbike crashed into a car outside a US military base in Northamptonshire in August last year.
Sacoolas, the wife of a US intelligence official based at RAF Croughton, claimed diplomatic immunity following the crash and was able to return to her home country, sparking an international controversy.
On April 30, a spokeswoman for the US State Department reiterated its position that at the time of the accident, and for the duration of her time in the UK, the driver had immunity from criminal jurisdiction.