A kilogram of heroin arrived in the Co Antrim town of Ballymena every month in 1997, police believed in 1998.

A “subculture of heroin users” was also present in South Belfast in the late 1990s, according to officials in previously secret state file titled Drugs – Heroin.

Officials commissioned research into the emergence of the drug in Northern Ireland in 1998 to monitor levels of usage.

Notes in the file indicated a correlation between heroin use and areas without strong community cohesion.

Information given at a seminar on March 27, 1998 included figures from the RUC that heroin had been present in Ballymena since the early 1980s, that one kilogram was arriving in the town every month during the previous year, that one gram of heroin costs £80-100 and that a third of a gram sold for around £35.

However, the level of usage of heroin in Northern Ireland overall was termed as low level compared to the Republic of Ireland, with 34 grams of the drug seized in Northern Ireland in 1994 compared with 5,672 grams in the Republic.

Another paper in the file said the “close knit nature of communities in Northern Ireland with strong family ties has prevented the development of widespread heroin abuse”.

It also noted paramilitary violence and intimidation directed against use of hard drugs such as heroin, although it added there is evidence of paramilitary groups being involved in the supply in other sorts of drugs.

Ballymena was described as having “pockets of deprivation” linked to high levels of drug misuse on estates such as Ballykeel and Doury Road where there was high rates of unemployment.

It recorded that heroin misuse started in the Doury Road estate where there was a history of drugs, including LSD and cannabis in the 1960s-70s, and ecstasy and amphetamines in the 1980s with the development of the rave culture.

“In 1994/5 two dealers began to obtain supplies of heroin which they made available in the Ballymena area through their local distribution network,” it was noted.

“There was a growth in heroin use and crack cocaine, especially among young unemployed males in the area, with dealing taking place openly on the estate.”

It was attributed to social disadvantage, low employment levels, lack of paramilitary influence, limited community structures and facilities, the impoverished nature of the area, absence of strong policing and the good infrastructure and close proximity to ports and airports which made the transportation of drugs easier.