A former chief constable of the RUC argued against the reintroduction of the death penalty in Northern Ireland in 1983.
Sir John Hermon said it was unlikely to deter terrorists claiming political motivation.
The exchange was revealed in the 1990 official files made public at the Public Records Office Northern Ireland (PRONI). The issue had been revisited by the Government that year, with then chief constable Sir Hugh Annesley reiterating his predecessor's advice about the impact of the sentence.
Sir John wrote: "They are much more likely to see and use the 'death penalty' as a vehicle by which to extend their political influence and to attract support both nationally and internationally to their 'cause'."
Sir John led the force from 1980 to 1989.
He wrote: "No sound reason can be found for the re-introduction of capital punishment that could be supported at the present either by logic or argument.
"The main perpetrators of these crimes are terrorists claiming political motivation and they are most unlikely to be influenced by any fear of death as a legal consequence of their actions."
The negative effects on security in Northern Ireland of introducing the capital sentence had been acknowledged by Sir John in a report he prepared for Government in 1983.
Seven years later the NIO revisited the issue and sought the advice of Sir Hugh.
Sir John said many terrorists were on the fringe of 18 years old, possibly too young for the death penalty.
"The death penalty could be avoided by increasing the 'use' of young persons by terrorist organisations," he added.