When the police began investigating Dr Harold Shipman in September 1998 they struggled to understand him.

How could a GP who was trusted and respected by more than 3,000 patients also be a killer who struck time after time with no obvious motive?

The house in Nottingham where Shipman was born

Harold Frederick Shipman was born into a working class family in Nottingham on 14 January, 1946.

Fred, as he was known, was a confident and clever child who was accepted into the local grammar school.

When he was 17 his life changed dramatically. His mother, Vera, died of lung cancer at the age of 43. For the first time Shipman saw the influence of doctors - administering drugs like morphine to alleviate pain - in the last days of a life.

Shipman as a teenager at High Pavement Grammar School

The teenage Shipman was ambitious and academically successful - and in 1965 he went on to study medicine at Leeds University.

But his life soon became more complicated when his 17- year-old window-dresser girlfriend, Primrose, became pregnant.

The two had first met after Primrose's father rented Fred a room. They married in November 1966 and moved into a flat. Together they had four children - all of whom are now adults.

Drug addiction

In 1970, Shipman graduated from university and started working at the Pontefract General Infirmary. By 1974 he had become a GP working in a practice in Todmorden, but he soon began to have blackouts.

It was at this time that his colleagues made the shocking discovery that Fred Shipman was addicted to the morphine-like drug pethidine.

Happy times for Shipman as a student

Shipman was convicted of making out drug prescriptions to himself and given a heavy fine. He was also fired from his job at the Todmorden practice.

He left town for a psychiatric and drug treatment centre in York and although his career was damaged, he was not struck off. Shipman's only explanation was that he had become fascinated with drugs while at college.

The senior partner at the Todmorden practice, Dr Michael Grieve, said: "If Fred hadn't at that point gone straight into hospital, perhaps his sentence would have been more than just a fine. I think it's perhaps the fact that he put his hand up and said 'I need treatment' and went into hospital, and then the sick-doctor routine takes over."

Hard working GP

Caring Shipman: the doctor's practice wins an award for raising money for charity

Sometime later in 1977, Shipman re-emerged as a GP in Hyde. His new colleagues respected his work, although some felt he could be arrogant and patronising towards his patients.

In 1993, he set up on his own, having fallen out with his partners. His wife Primrose worked as a part-time receptionist and the new practice attracted a large number of patients.

But on 7 September 1998, his world came crashing down when he was arrested and charged with the murder of Kathleen Grundy.

As police investigated they uncovered evidence of a further murders.

Controlling and dominating

DCI Mike Williams: "He was arrogant"

During their interviews with him a highly confident Shipman denied all charges.

Detective Chief Inspector Mike Williams said: "He was an arrogant type of individual to deal with. And I don't say that lightly.

"I've listened to the interviews, and he certainly wanted to control and dominate the interview and the officers, at times belittling them. He was treating this as some sort of game, a competition, pitting his, what he considered to be his superior intellect, to those of the officers who were interviewing him."

His crimes defy easy answers. In only one murder - that of Mrs Grundy which led to his downfall - did money appear to be a motive.

Perhaps Shipman was an ambitious man who, disappointed by life, struck out at middle-aged women for some complex psychological reason.

Psychologist Richard Badcock interviewed Shipman

Forensic psychologist Dr Richard Badcock, who interviewed Dr Shipman in an attempt to provide the police with an insight into why he had killed, told the BBC: "He was very definitely not doing it for excitement, far from it.

"He was doing it mainly to try and resolve something within himself...to get rid of an anxiety but an anxiety which he might not even have let himself think about."

The South Manchester coroner, John Pollard, who knew and worked with Shipman, has his own theory about the doctor's motives.

"I think the only valid possible explanation for it is that he simply enjoyed viewing the process of dying and enjoyed the feeling of control over life and death, literally over life and death."