The Post Office engaged in a campaign of threats and deceit in an attempt to suppress evidence that would have cleared postmasters involved in the Horizon scandal, according to a BBC Panorama investigation. In 2015, the programme featured an interview with former Fujitsu insider Richard Roll, who revealed that accounts on the Horizon computer system used by postmasters could be altered without their knowledge. The Post Office sent intimidating letters to experts who spoke to the BBC and also claimed that remote access to the system was not possible, despite having been warned to the contrary four years earlier.
https://www.bbc.co.uk/news/uk-67884743