in 1961, aged just 16, Angus Robertson Sinclair stood in the High Court in Edinburgh and admitted killing his seven-year-old neighbour, Catherine Reehill.

Now 69, he was convicted yesterday of another two murders, officially his third and fourth, that of Helen Scott and Christine Eadie, in 1977. They were both 17.

Police believe he has killed and maimed many more women and children.

All but one of his proven and suspected crimes were carried out in 14 years from 1968, the short period of his adult life when he was not behind bars.

Jailed for life for rapes in 1982, he has spent most of his life in prison and there can be little doubt that is where he will die because he will be aged 106 before he can apply for parole.

Now a white-bearded pensioner, it is hard to imagine he was once a teenager whose own lawyer thought he could not be cured of his passion for killing.

Gerald Boyle, back in 1961, said he had been "appalled to learn from a medical report that no amount of psychotherapy was likely to benefit him ... his conduct after the body was found was so normal as to indicate a great degree of abnormality".

More than half a century later, in another court, Sinclair, again behaved as if everything was normal. He looked uninterested, demonstratively yawning, as he heard evidence over the last five weeks at the High Court in Livingston of his 1977 murders.

At 5ft 3in, he was towered over by his minders in court.

Sinclair was always small. Born in 1945, he was brought up in a tenement in St George's Cross, Glasgow, by his mother Maimie, along with a brother John and sister Connie.

His father, also Angus, died when he was four. Years later, after her son admitted killing Catherine Reehill, Maimie speculated the death of his father may have damaged Sinclair mentally. "Angus was so young at the time, but he missed his father terribly," she said. "I keep wondering if that could have anything to do with what happened."

Those who remember him say he was an introvert, largely shunned by other children. Don Stuart, who lived in St Peter's Street during the late 1940s, described him as a "wee smout".

Maimie, who stood by her son until she died in 1987, remembered a very different boy. After he admitted killing Catherine, she said: "The Angus I knew was a good son. He was a wonderful boy at home, but something happened to him. He seemed to change three years ago. I began to feel a bit frightened and worried about him."

The change was puberty, and with it came the onset of Sinclair's morbid obsession with sex and violence. Shortly before his 13th birthday, he started at St George's Road Secondary. His time there was deeply unhappy: boys bullied him and girls ignored him. He was a failure socially and academically. One psychiatrist who examined him said he was "not a simpleton, but below average intelligence".

As soon as he was legally allowed, he left school and got a job as a van boy. That was the summer of 1960, his last of innocence. Six months later, an eight-year-old girl was indecently assaulted nearby.

Peggy Reehill, 84, the aunt of the first girl killed by Sinclair, speaking in 2007, said: "He was caught interfering with wee lassies in Cowcaddens, people knew that about him at the time."

Sinclair was arrested and charged with lewd and libidinous practices but, because he was only 15, he was placed on probation for three years. It was not enough to deter him.

He killed Catherine on Saturday, July 1, a sweltering day. Sinclair was home alone when he saw her skipping down the street. She, her six-year-old sister Margaret and brother Jim, five, were staying with their aunt while their parents, Patrick and Vera, were in London looking to begin a new life.

Sinclair approached Catherine and gave her money to go to the shop around the corner. When she returned, the teenage boy pinned her against the wall and "interfered" with her. She screamed for her daddy. The boy strangled her with a bicycle inner tube, then dumped her body in the close.

She was found on the basement steps by two women going to the bingo. Within a minute, Sinclair appeared and volunteered to call an ambulance, telling the operator Catherine had fallen down the stairwell. There was blood on her nose and a large bruise on her forehead. She died in an ambulance.

A murder investigation was launched. Sinclair had disappeared and this was noticed by the neighbours. He was arrested and charged the following day.

On Monday morning, Sinclair, who was 16 just 24 days previously, appeared in the juvenile court in the old Partick Marine police station, as Catherine's parents arrived in Glasgow. Maimie later claimed he showed remorse while on remand.

The following month at the High Court in Edinburgh he pleaded guilty to the culpable homicide of Catherine and was sentenced to just 10 years.

Sinclair was out in less than seven, his "abnormality" having provoked leniency rather than harshness in the court. His 14 years of rape and murder began.

Retired detective chief inspector Allan Jones, 52, believes Sinclair is responsible for another nine killings, six unsolved. This would make him one of Scotland's most prolific serial killers.

Mr Jones said: "It is a unique modus operandi. It was people being abducted from a public place. It was improvising with the tights. That MO didn't fit anyone else in the UK." The security adviser added: "He had a caravanette and was always going on fishing trips."

Operation Trinity, which examined a series of unsolved murders of young women in Scotland, identified Sinclair as the chief suspect in the World's End murders of October 1977, as well as the rapes and murders of Anna Kenny, 22, in August, Hilda McAuley, 36, in October and Agnes Cooney, 23, in November of the same year.

Ms Kenny, Ms Cooney and Ms McAuley all disappeared in Glasgow; in all of the cases, the victims were bound and gagged in the same way.

One of the FBI's leading criminal profilers said the murder of Frances Barker, 37, in Maryhill was "uniquely similar" to the other killings, although another man, who has since died, was convicted of that crime.

The murders did not end in 1977. The next year Mary Gallacher was strangled with an item of her clothing after being raped in a Glasgow lane. Sinclair was convicted of her killing in 2001.

Mr Jones also believes Sinclair is responsible for the death of Eddie Cotogno, 63, an amateur pornographer and friend of Sinclair, in Glasgow in 1978, and Helen Kane, 25, whose body was found on a hill overlooking Holyrood Park, Edinburgh, in 1970.

In a leaked 2007 letter Sinclair said he could be questioned over the murder of Patricia Black, 22, who was last seen boarding a bus to Saltcoats from Irvine in 1976 and Sandy Davidson, a three-year-old last seen running from his garden in Irvine the same year.

Sinclair had married in 1970 - to Sarah Hamilton, sister of Gordon Hamilton, the man, now dead, with whom he killed Ms Scott and Ms Eadie in 1977.

Sinclair and his wife had a son, Gary, and Maimie Sinclair thought her son's evil days were over. Sadly, they were not.