Detectives based at Mountjoy Prison are investigating the circumstances surrounding the death of 20-year-old Anthony Kelly in St Patrick's Institution on 31 May 2006.

It had initially been thought that Kelly, from Darndale in North Dublin, had died after a drugs overdose. It has now been determined that the cause of death was not in fact an overdose but that the highly toxic substance strychnine actually killed him.

Strychnine is an extremely potent poison that is commonly used to kill rats.

Detectives are examining if the poison was deliberately injected or administered to the dead man or if any drugs he may have taken contained rat poison.

Garda� launched the investigation following a request from Dr Brian Farrell, the Dublin county coroner. Farrell took the decision after examining the post mortem report by state pathologist Dr Marie Cassidy.

Farrell said: "A serious issue has arisen in relation to the autopsy report.

My concern is that the cause of death is given as strychnine poisoning and I am not at this moment in a position to say how this came about."

Anthony Kelly was found unconscious in his cell and was rushed to the Mater Hospital, where he later died.

A garda source said of their investigation: "It looked like this case was simply as a result of an overdose but the toxicology report found large amounts of strychnine in the system which complicates things greatly.

"Strychnine is not the type of substance that you can just pick up on the street, it has to be ingested or administered into the body. We believe at this moment that the most probable way this happened was that heroin was smuggled into the jail but it was a bad batch.

"Every so often you will find that all sorts of stuff is mixed in with heroin and rat poison is often one such substance.

Rat poison mainly contains strychnine and if it can kill a rat, with its immune system, you can only imagine how potent it is.

"Anthony Kelly must have been in considerable pain before he died. He would get very thirsty and if he drank water it would only have made things worse.

"It is a tragic case but we have to investigate in case it turns out that somebody poisoned him deliberately - but we have no evidence of this at the moment, " the source added.

Kelly's family have declined to speak about the circumstances of their son's death. It is expected that the garda investigation will take a number of weeks to complete and the findings will be forwarded on to the county coroner.