Brian Murphy and David Langan, the Dublin teenagers beaten to death in the past two weeks, appear to have been the victims of an outbreak of violence among middle-class schoolboys that has been causing concern to both parents and gardai.

Students who frequent the same discos and bars that the two youths had visited on the nights they died all speak of a culture of violence, particularly among school rugby players, and of heavy drinking.

On August 31st, the night that Brian Murphy was killed, hundreds of pupils and ex-pupils from fee-paying Dublin schools gathered at Club Anabel in the Burlington Hotel for an end-of-summer disco. During the evening there were outbreaks of taunting and threats between boys from two rival rugby-playing schools. Doormen at the disco had to restrain a number of youths, and one was carried, struggling, from the disco.

A student who attended the disco said there was aggressive behaviour throughout the night between rival groups of "jock rugby types". There was also excessive drinking. On one visit to the toilets, this youth saw two other boys simultaneously being sick into the urinals.

During the evening the club served cheap drink between 10.30 and midnight. All drinks were £2. Statements taken by gardai confirm the extent of the drinking. One youth questioned on the first day after the death of Brian Murphy said he had drunk several pints of lager and then had four or five shorts, but had no idea what type of spirits were in the glasses.

Cheap drink promotions are a feature of club life. One disco in the city serves measures of vodka and the caffeine-enriched soft drink, Red Bull, for £1. This would normally cost around £5. In another venue, all drinks are £1 on one of the mid-week disco nights.

A doorman working at one of the leading discos said the cheap drink is almost always followed by violence. "We see fights every night, often two or three fights in a night. On nights when the drinks are £1 and they are doing shorts with a dash of Red Bull, they kick lumps out of each other."

Fatalities resulting from brawls outside pubs and discos are a feature of the Garda Siochana's annual homicide list. Four such killings took place this year before the deaths of Brian Murphy and David Langan. However, in the previous four instances, the victims were men in their 30s or 40s. It is a common feature of such cases that both the victims and their attackers tend to be in poor physical condition, and to have been drinking heavily for prolonged periods before the fatal encounters.

A teenage girl who frequents the disco at the Palace in Dublin, which David Langan had attended before he died, said: "You know it is the start of the rugby season. There is always aggravation. We leave before the end because, inevitably, there will be trouble outside. The discos are very well run inside."

Dublin disco doormen are aware of the potential for trouble at the end-of-summer student discos, when youths unaccustomed to drinking pour out on to the streets at around 3 a.m. It had been expected there would be less trouble this year. Large groups of youths - about 340 in all - from three rugby-playing schools were out of the city on package holidays in Tenerife, accompanied by two large groups from girls' fee-paying schools.

The students in Tenerife were keeping their friends in Dublin informed of the goings-on by mobile telephone. Students said there were outbreaks of fighting between the three boys' schools in the holiday resort.

A teenager who knew Brian Murphy and who was interviewed by gardai in the days after his death told detectives that a month previously he had an argument outside another disco with youths from one of the schools on holiday in Tenerife.

Concerns have been expressed that the aggressive behaviour among pupils in rugby-playing schools might be related to the use of performance-enhancing drugs. It is suspected that these anabolic steroids and other associated drugs are widely used by young rugby and Gaelic football players.

One of the most common legal performance-enhancers is still advertised in sports magazines. The active ingredient, creatine, gives young athletes increased energy bursts, but it is suspected that it also contributes to increased aggression known in sports circles as "roid (short for steroid) rage".

This increased power and aggression is also seen as contributing to more serious injuries on the sportsfield. In the last rugby season, three young players were seriously injured, one a Dublin schools player who suffered a catastrophic injury leading to paraplegia.

Sources close to the investigations into the deaths of Brian Murphy and David Langan report that both appeared to suffer extraordinary powerful blows to the head. Gardai have also commented that there does appear to be unusually aggressive behaviour sparked by very minor arguments.

In both investigations, detectives are gathering evidence and taking statements from large numbers of witnesses and people who were present on both nights. There has been no rush to arrest suspects, as in both cases the identities of the youths are known and it is considered very unlikely that any would flee the jurisdiction.

When there is a sufficient accumulation of evidence, detectives are expected to make arrests. This may not be for weeks. In the case of Brian Murphy, it is likely that five youths, all ex-pupils of a leading rugby-playing, fee-paying Dublin school in the south of the city will be questioned.

In the case of David Langan four youths from two schools in the north of the city will be questioned. According to Garda sources, all come from affluent backgrounds.

Files on both cases will then be submitted to the Director of Public Prosecutions who will decide if there are grounds in either case for serious charges.