The equation is simple: It had to be either Robin or David.

Now it's time to make a decision.

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Despite giving evidence in court only once, David Bain has said a lot about the shootings in various contexts.

He has steadfastly maintained his innocence but the details of his story have changed. How plausible is it?

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If Robin murdered his wife and children, his final task would be to shoot himself. But how likely is it that a right-handed man would shoot himself in the left temple? Is it even possible?

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Could Robin have murdered his children and then taken his own life? Robin had blood and bruising on his hands. His foot size was more likely to fit bloody footprints in the house. Was a mysterious note left on the family computer Robin's final words?

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Although police initially regarded the shootings as a murder/suicide, evidence began to mount against David Bain.

His fingerprints were on the rifle, he had blood on his T-shirt, a lens from glasses in his bedroom were found in his dead brother's room and he had injuries that were consistent with a fight.

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Motive has always been the most puzzling aspect of the Bain shootings.

Neither Robin nor David seemed like mass killers. But both had reasons to kill.

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The shootings appalled the country and deeply shocked the community and wider Bain family.

Yet sole survivor David Bain's unusual reaction surprised many. Loyalties were tested. Suspicions were raised. And then an arrest is made.

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From the outside, the Bain family seemed to have settled into a new life in Dunedin.

Eldest son David was on a new path, while sister Laniet appeared to have given up prostitution and Arawa was thriving as a budding teacher.

But in the six months before the shootings, cracks began to emerge.

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By any standard, this family was unusual, even bizarre.

Mother Margaret Bain's complicated spiritual life and beliefs meant the family was constantly encouraged to put Satan behind them.

She practised self hypnosis, meditation and channelling. Their life in Papua New Guinea helped to concentrate the family's oddness. How did it influence the slaughter?

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It's June 20, 1994. Police are called to a rundown home in Dunedin, New Zealand. They're not sure what to expect after a panicked phone call.

But inside were five dead bodies, a chaotic scene and and several clues pointing to the killer.

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