One email showed an official commenting on Charlotte Charles, the teenager’s mother, and describing her language as “excessive”, while the family’s spokesperson was labelled “evil”.
The documents were obtained by the family after they submitted a subject access request to the Foreign Office and Home Office.
Ms Charles had said "you wouldn't treat an animal the way government treated us", after a senior diplomat at the Foreign Office (FCDO) sent a message to their US Embassy counterpart saying they should "feel able" to put suspect Anne Sacoolas on the next flight home.
In response to her comments, one official wrote: "Even aiming for someone bereaved that line about being 'treated like animals' is totally excessive!"
Mr Dunn, 19, was killed when his motorbike crashed into a car outside RAF Croughton, a US military base in Northamptonshire, on 27 August 2019.
His alleged killer, 43-year-old US citizen Ms Sacoolas, whose husband is a US intelligence official based at RAF Croughton, claimed diplomatic immunity following the crash and returned to the US, sparking an international controversy.
The family has been represented by spokesperson Radd Seiger since September 2019. The documents show officials at the Foreign, Commonwealth and Development Office (FCDO) have described him as "evil", "aggressive" and "erratic".
One email said: "Everything the family say is completely wrong. The trouble is they don't understand anything at all of the wider context - and the evil Radd Seiger has not explained it to them...
"And so convinced are they of everyone's bad faith that it never crosses their mind that anything of that we are telling them might be true."
The permanent under-secretary for the FCDO, Sir Philip Barton, sent the family a letter of apology in which he described the emails as "isolated comments".
He wrote: "As you will see, the digest contains some unprofessional and unacceptable language that you may find upsetting. I want to offer my sincerest apologies for the hurt that this may cause.
"I also want to emphasise that these isolated comments are from a handful of officials and do not in any way reflect the wider views of the FCDO or the core team working with you to deliver justice for Harry.
He added that “appropriate management action has been taken” against the emails’ authors.
Mr Seiger criticised the involved officials’ “unprofessional conduct", telling the PA news agency: ”It is appalling to see that whilst we were seeking justice for Harry as anybody would, these officials thought it was appropriate to engage in baseless keyboard warfare."
He added: "I am more than happy to bear the brunt of any criticism, but to attack a bereaved, grieving mother who has never put a foot wrong in the campaign is scandalous."
Lisa Nandy, the shadow foreign secretary, condemned the “utter disrespect for a bereaved family who are rightly fighting for justice for their son".
Ms Nandy tweeted: “To describe a grieving mother’s feelings as ‘excessive’ is simply inexcusable. From the outset, Foreign Office ministers have refused to level with the family and repeatedly denied them vital information about how Anne Sacoolas was allowed to leave the country. To insult them and then try to spin their way out of it is beyond contempt.”
She called for the foreign secretary to apologise in person at the House of Commons tomorrow and “outline the actions he will be taking to finally secure justice for Harry Dunn”.
An FCDO spokesperson said: ““We are very sorry for the offence caused to the Dunn family and their spokesperson. The views expressed by a small number officials were deeply inappropriate, and in no way represent the views of the Foreign, Commonwealth and Development Office.
“We will continue to do everything we can to support the family to achieve justice for Harry.”
Watch the latest from Yahoo UK News