During sentencing in 2015, Judge Clement Goldstone said: "They decided that Anthony Duffy had to be eliminated, not only because he was a rival drug dealer, but even more because he dared to cross them and the message had to be sent out as to who was boss.
"Anthony Duffy has paid the ultimate price. His family's loss is unquantifiable."
Neither of the prison guards were hurt during the incident that took place after Walmsley's appointment.
A manhunt is now under way.
The public are being warned not to approach the killer, described as dangerous, as he could also be in the company of the armed men who ambushed the car.
- Originally published by Telegraph UK