The parents of a motorcyclist killed in a crash say they will only meet the US woman allegedly involved if she promises to return to Britain.
Harry Dunn, 19, died in a crash near RAF Croughton, Northamptonshire.
Anne Sacoolas left the UK under diplomatic immunity while police were investigating. She has offered to meet Mr Dunn's parents, who are in the US.
A Dunn family spokesman said the parents' pre-condition for a meeting was a "hurdle" to it taking place.
Radd Seiger told the BBC the parents, Charlotte Charles and Tim Dunn, who have travelled to the US, were unlikely to meet Mrs Sacoolas this week.
"Mrs Sacoolas has to commit to returning to the United Kingdom to submit herself to the English authorities, to Northamptonshire Police, and to co-operate with their inquiries," he said.
Her return was "a non-negotiable red line", he told Sky News.
The parents hope to gain media exposure in the US, to put pressure on President Donald Trump "to send Mrs Sacoolas back", he said and have been involved in a round of media interviews on Monday.
Ms Charles told the BBC's Duncan Kennedy they had received messages of support from people in the US "probably in their thousands", and similar messages from "all around the world".
"I think everyone can see she's not done the right thing and she needs to do the right thing. She should have just stayed. It should not have come to this. It's ludicrous," she added.
Speaking on CBS This Morning, Mr Dunn described how he spoke to his son for the last time as paramedics loaded him on to a stretcher by the roadside.
"I could see broken bones out of his arms and stuff. He was talking. He knew [that I was there]," said Mr Dunn.
"I called over to him and said 'Harry, it's your dad - they are going to fix you. Be calm. Let them help you'."
Later Mr Dunn told a press conference: "I've always wanted to ask her if she could explain the moment of the crash. Find out if she comforted Harry. If she spoke to Harry. Find out what her movements were. Did she try and call the emergency services?
"I'm just struggling because I can't imagine my lad being in the ditch and not having any comfort from anybody until the ambulance and police turn up 'X' minutes later."
Ms Charles added: "We're not inhumane, we still don't wish her any ill harm but we need to hear it from her, in her own words, in a room, on our terms, in the UK with therapists and whoever else can help us, mediators.
"But just hearing it through a statement...we're seven weeks in now, it's a bit too much too little too late, I'm afraid."
At a press conference last week. a briefing note held by Mr Trump at the press conference appeared to suggest Mrs Sacoolas would not be returning to the UK.
Mr Dunn said: "I would say to him (President Trump) as a man, as a father, how could you let this happen, if you are a father and your child died surely you'd want that person to own up and take responsibility for their action?"
Mr Seiger said on Radio 4's Today programme the weeks since the teenager's death had been "a very, very dark time" for the family and that "every second that passes is another second of pain".
Discussions over Anne Sacoolas's potential extradition from the US are likely to be a "delicate interplay" of legal obligations and political realities, says an expert in international law.
Mrs Sacoolas's case appears to meet the conditions agreed in the US-UK extradition treaty in force since 2007, said Prof Tarcisio Gazzini from the University of East Anglia. Given that she is no longer in the UK, diplomatic immunity no longer applies, he said.
Prof Gazzini predicted the process was likely to end in one of two ways - Mrs Sacoolas agreeing to return to the UK to face prosecution; or the US extraditing her in accordance with the treaty, and waiving her immunity. It is unlikely she would return to the UK to reinvoke diplomatic immunity, he said.
Prof Gazzini said: "My guess is that the US will discuss this with the lady and say that they are prepared to allow her to be tried in the UK.
"The two governments would prefer to go through the treaty, and preferably through Article 17 [where she agrees to be surrendered].
"Assuming all the conditions are satisfied and documents are in order then the US is obliged to [extradite]. If not, they commit a breach of international law."
Mrs Sacoolas is reportedly married to a US intelligence official.
A letter from her lawyers said she wanted to meet Mr Dunn's parents "so that she can express her deepest sympathies and apologies for this tragic accident".
Mrs Sacoolas, 42, was said to be covered by diplomatic immunity as the spouse of a US intelligence official, though that protection is now in dispute.
On Saturday, Foreign Secretary Dominic Raab wrote to Mr Dunn's family to explain that the British and US governments now considered Mrs Sacoolas's immunity irrelevant.
He said the matter was now "in the hands" of Northamptonshire Police and the Crown Prosecution Service (CPS).