In the early hours of March 7th last year a man broke into one of the row of patients' homes at Orchard View, adjoining the old Grangegorman Hospital in north central Dublin. Inside, he immediately acted strangely, not in the way a normal housebreaker might be expected to behave. He arranged bits of glass from the window he had broken and then collected almost every sharp implement from the kitchen drawers.

He made his way upstairs to the bedrooms of two long-term psychiatric patients, Sylvia Shields and Mary Callinan, aged 59 and 61 respectively, and stabbed both women to death.

His first victim was still asleep and killed almost instantly with stabbing blows of such force that at least one of the murder weapons was found embedded in the body. Her body was found where she had been sleeping.

His second victim, awakened by the commotion in the adjoining bedroom, was out of her bed when she was struck by the first of a series of savage stabbing and gashing blows. She died on the floor of her bedroom.

The murderer then approached another bedroom, where the third resident, another elderly psychiatric patient, was sleeping. He looked into the room, where she was still asleep, then left the house without touching her.

The murders of Sylvia Shields and Mary Callinan were unlike any other murders in the State in living memory. There was no known motive or reason why they should have been killed.

These were random murders committed by a man in a killing frenzy. A forensic psychologist brought in from England to help the investigation warned that the man would probably murder again, given the chance.

The fact that gardai had their first potential random multiple, or serial, murderer spurred them into mounting the second-largest investigation since the start of the Veronica Guerin murder case.

A large team, at one stage involving more than 200 officers, was assembled in the Bridewell station in Dublin to pursue the Grangegorman killer. A list of hundreds of potential suspects was assembled. Nothing happened until the end of July. Then one of the heroin addicts who drift around the Smithfield and Grangegorman areas, where there are hostels and derelict buildings to shoot up in, brought gardai a story about a man who had allegedly spoken about the killings.

On the morning of Saturday, July 26th, dozens of gardai, including several armed officers, surrounded the Salvation Army night hostel for young homeless men in the grounds of the derelict St Brendan's Hospital at Grangegorman, a few hundred yards from the house where Ms Shields and Ms Callinan were murdered.

The hostel is in an abandoned recreation hall built in the middle of this century for the nursing staff at Grangegorman. With rows of single beds, it is filled each night with homeless young men, most of them heroin addicts.

The gardai burst in and seized Dean Lyons, a 24-year-old addict, as he was walking in one of the partitioned dormitories before leaving the hostel for the day.

He was taken to the Bridewell Garda station. That afternoon he was questioned about the murders in a detention room in front of a video and audio recording machine. He made non-committal answers, it is understood.

He was questioned later on Saturday evening. As a result of a statement which emerged from this period of questioning gardai recommended to the Director of Public Prosecutions that Dean Lyons be charged in relation to the Grangegorman killings. The next day he was charged with the murder of Ms Shields.

He appeared in Dublin District Court on the following Monday morning for his first public arraignment on the charge. The court remanded him in custody to Mountjoy Prison, where he has been since.

After the initial flurry of interest in the bizarre and horrible double murder, media attention died down as the case effectively became subjudice and potentially a legal risk to report.

Mr Lyons was accustomed to arrest and imprisonment. He had been a drifting heroin addict in Dublin for three years since leaving his family home in Castlepark, Tallaght.

Two days after the murders in Grangegorman he was arrested under the public order laws in the Kilmainham area after he was seen apparently taking heroin on a vacant site.

A month or so later he was arrested by gardai from Pearse Street station on suspicion of being involved in an attempted syringe robbery in the city centre.

On neither occasion was there apparently any suspicion on the part of the gardai that Mr Lyons was connected in any way with the murders.

Mr Lyons has a swarthy complexion, and is slightly built and of medium height. He was well known to the rootless community around the Smithfield-Grangegorman area.

People who know him describe him as a friendly, feckless young man whose only interest was in acquiring heroin. He has had no steady relationship with anything other than his heroin habit.

He had, according to one of the few apparently sensible people who knew him, a habit of seeking to be the centre of attention and was prone to telling stories about himself.

One man who had known him for the two years before his arrest pointed out that after two years of heroin abuse he had become, like all persistent abusers, weakened by the drug's suppression of his metabolic system. Those who knew him said he was not a threatening figure in any way.

There was some consternation and surprise among those who knew him when he was charged. One man who knew him in his professional capacity as a carer said he believed from the outset that the gardai were making a mistake.

No one who knew him, including his family, is prepared to speak publicly about the affair.

After he was charged it appeared that the machinery of the criminal justice system would lead him forward to a second charge, that of also murdering Ms Shields, trial, and if convicted, a sentence of life imprisonment.

That is, until mid-August last when an incident occurred outside Dublin which set in train a process which has led to a realisation that Mr Lyons is innocent and which, next Wednesday, is expected to see the State reverse its position and withdraw the murder charge against him.

As a result of a serious crime outside Dublin in August, gardai unconnected with the Bridewell investigators were given a confession from another man that he had killed Ms Shields and Ms Callinan.

This statement and the circumstances surrounding the serious second crime caused immediate concern among many gardai, although no legal moves were made or any public statement issued.

The existence of the second man's statement was first publicised, 10 days later, in The Irish Times of August 26th. Again there was no public comment from the authorities.

Unnamed garda sources were quoted on radio, television and in other newspapers, dismissing the Irish Times report and the significance of the second statement.

That statement, according to one of the anonymous sources, was "riddled with inaccuracies". Senior detectives were said to be "highly sceptical" of its worth.

An internal inquiry was set up by the Garda Commissioner, Mr Pat Byrne. Led by Assistant Commissioner Jim McHugh, the officer in charge of the Dublin Metropolitan Area, and assisted by two senior detectives, the inquiry reached conclusions in the past six weeks which have led to the DPP's decision to withdraw the murder charge against Mr Lyons.

The charge will be withdrawn, it is understood, when he makes his next remand appearance in the District Court on Wednesday. There is still no official comment about the case, and few sources are prepared to speculate if there will be any further action arising from the fact that an innocent man was charged with murder.

There is also, still, no comment on the fact that while an innocent man was in custody the real murderer has yet to be charged.