The family of Harry Dunn have revealed they will fly to the US on Sunday to pursue justice for their son, as Boris Johnson said Washington was “absolutely ruthless” in its safeguarding of its diplomats and their families.

Anne Sacoolas, the wife of a US intelligence officer, fled the UK after allegedly killing 19-year-old Dunn in a road collision. Dunn died near RAF Croughton in Northamptonshire on 27 August.

In a post on the GoFundMe page, Dunn’s family said: “We are overwhelmed by the generosity and the support from all of you and want to say thank you again personally from the bottom of our hearts.

“We couldn’t do this without you and value each and everyone of you! The individual comments and personal stories that have been coming into us have touched our hearts and we know that we are not alone.

“We wanted to share with you that on Sunday we will be flying to the US to continue our fight for justice4harry.”

The family plans to meet various news channels to increase awareness of their campaign to get the US government to send Sacoolas back to the UK.

The Foreign Office doubts 42-year-old Sacoolas will return to the UK to answer questions about her role in the crash, even though some legal experts have challenged the US claim that she is entitled to full diplomatic immunity.

Briefing notes for a press conference given by Donald Trump on Wednesday show the Foreign Office is right to be pessimistic about Sacoolas’ return. The note, caught on camera by a Washington Post photographer, told Trump: “(If raised) Note, as Secretary Pompeo [the US secretary of state, Mike Pompeo] told Raab [the UK foreign secretary, Dominic Raab], that the spouse of the US government employee will not return to the United Kingdom.”

Before she left the UK, Sacoolas had been helping British police with their investigation into the collision between Dunn’s motorcycle and her Volvo XC90. They believe she drove on the wrong side of the road for 400 metres after leaving the base.

Johnson has called for Sacoolas to face justice in the UK. The prime minister said: “In my experience, America is very, very reluctant to allow its nationals to be tried overseas, and is absolutely ruthless in enforcing the code of diplomatic immunity.”

Johnson said it was not appropriate for Sacoolas to use her husband’s diplomatic immunity in this case. “I made that point to the president and he is sympathetic,” he said. “I think we’ve just got to keep working on that and see what we can do to get justice for Harry Dunn and his family.”

Raab met Dunn’s mother, Charlotte Charles, and his father, Tim Dunn, on Wednesday afternoon after speaking with the US ambassador Woody Johnson.

Charles said afterwards: “I can’t really see the point as to why we were invited to see Dominic Raab. We are no further forward than where we were this time last week. Part of me is feeling like it was just a publicity stunt on the UK government side to show they are trying to help.”