A notorious rapist due to be released from prison next month is regarded by gardaí as the prime suspect in the disappearances of three woman, a former garda commissioner has confirmed to the Sunday Tribune.

Larry Murphy (45), a carpenter from Baltinglass in Wicklow, was convicted of the abduction and repeated rape of a Carlow woman in February 2000. He is due for release from Arbour Hill prison on 11 August.

Operation Trace was established by retired former garda commissioner Pat Byrne in 1998 to investigate the disappearance of six young women who all vanished from the Leinster area between 1993 and 1998.

Its objective, aside from solving the cases, was to try and determine if a serial killer was responsible for some or all of the suspected murders. Detectives from Operation Trace have interviewed Larry Murphy on at least three occasions about the missing women but he has consistently denied any involvement.

Pat Byrne, who retired from the force in 2003, said: "In the circumstances of the crime for which he was convicted, that was a terrifying ordeal. People must reach their own conclusions about Larry Murphy. After Murphy was arrested, he became a suspect in some of the other cases of women who have disappeared."

The former commissioner said that one of the "major disappointments" of his career was failing to solve the cases of the six women who disappeared in the Leinster area: Deirdre Jacob, Jo Jo Dullard, Annie McCarrick, Ciara Breen, Fiona Pender and Fiona Sinnott.

"The lack of a crime scene was a critical problem. There is no doubt those women are dead. There were times when we thought we were making progress, especially in relation to Larry Murphy. But a criminal investigation must be based on evidence and as the bodies have never been found, that was a major problem. Some people must know something and I would appeal to them to come forward."

Gardaí suspect Murphy of involvement in the disappearance of Deirdre Jacob, Jo Jo Dullard and Annie McCarrick. He has been effectively ruled out of the cases of Ciara Breen, Fiona Pender and Fiona Sinnott.

Murphy was attempting to murder the Carlow woman he abducted and raped in 2000 when he was intercepted by two hunters. He later pleaded guilty to the rape and abduction charges. He has refused to undergo any sex offenders' rehabilitation programmes. He will not be electronically tagged but will be monitored around-the-clock.