Gardai have not come across any of the type of Mexican "tar" heroin which has been connected with deaths of heroin addicts on the west coast of the US in the past year. These deaths bear similarities to the recent deaths in Dublin and Glasgow.

Most heroin reaching Dublin and Glasgow comes through Amsterdam and usually originates in Asia. However, Europol, the EU police intelligence agency, has reported that South American gangs are increasing their flow of drugs into European markets.

The two batches of heroin which were used by two of the victims and have been located by gardai from the Garda National Drugs Unit are understood to be in powder form. These samples are being sent to the US for examination.

Garda sources said it seemed uncertain whether all the deaths could be attributed to a contaminated batch of the drug. An average batch of heroin will be used by at least 100 or 200 addicts, and it might be expected that any bacteriological contamination would be more widespread.

However, it is understood that the victims were all injecting directly into muscle tissue, as the addicts all had problems from prolonged injection into their veins. This was also the case in the deaths, in similar circumstances, of addicts in the Oakland and Hayward areas of California.

After the Garda Commissioner appointed the drugs unit to uncover the source of the heroin used by the victims, officers visited relatives and associates but were able to trace only two samples of heroin which had been used.

In both cases the heroin appears to have been of unusually high strength. The drugs unit officers discovered that in all cases the victims appeared to have been diluting or "cutting" the heroin with citric acid.

Not all the Dublin victims have shown similar symptoms. Two who died from hypothermia and pneumonia and a woman who died from a ruptured spleen are likely to be ruled out of the investigation into a possible bacteriological source.

An investigation by detectives outside the drugs unit has begun into the death of the woman from a ruptured spleen, as it is now believed she may have been assaulted.

The woman is known to have had a number of disputes with other addicts and heroin dealers.