Only Perry Samuel knows exactly what happened on Bonfire Night 2006 when he murdered his young son and daughter.
It had been at the end of an otherwise unremarkable family day.
The apparently dutiful father was looking after his children while their mother went to a concert in Manchester and stayed over with friends.
After taking Caitlin, five, and Aidan, three, to McDonald's for tea, he brought them home, watched fireworks with them and gave his children a bath.
But he later rang the police, saying: "There's a problem with the children."
The problem was, he had suffocated them.
The unemployed shop assistant has admitted responsibility for their deaths, but despite being questioned for days by police, continues to refuse to say how, why, or even exactly when he killed them.
"On the face it, it had been a normal day," the police said later.
The police have called on Samuel to "do the right thing" and explain what happened that night, so the childrens' mother, Sarah, 23, is no longer tormented by endlessly imagining, and re-imagining, what might have happened.
Police called to the house found the children's bodies in the bath
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The couple were said to have had difficulties in their relationship, and had been separated, but at the time he turned to murder, Samuel and Sarah Graham were again living together in Bodelwyddan, Denbighshire.
The first the children's mother knew about her partner's dreadful actions was when she took a phone call at her hotel from the police, who then collected her to reveal the full horror of what her partner had done.
"This has been a harrowing case that has touched the hearts of all those involved in it," senior investigating officer Det Supt John Chapman was to tell journalists some months later.
Samuel had a history of mental illness. He had been diagnosed with bipolar disorder, or manic depression as it is also known.
Mold Crown Court heard he had been receiving different forms of mental health care since March 1998, when he was admitted to a psychiatric ward for a month.
The court was told he had been sectioned in January 1999 and arrested in November 2000 for an incident in which he admitted burglary. A charge of arson was ordered to lie on file.
In June 2004, he suffered a further manic episode and in, March 2006, was said to be in low mood during his separation from Ms Graham.
Tributes were left outside the house
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But contrary to some reports, he had not been in hospital immediately before the killings.
He has not used his mental health as an excuse for what he did - he pleaded guilty to murder, not manslaughter on the grounds of diminished responsibility.
His defence barrister Stephen Riordan QC said: "It's perfectly clear that this is a man who since 1998 has been in and out of psychiatric hospitals.
"He has been diagnosed with bi-polar disorder, which is a serious mental illness treated by prescription drugs and which is more generally known as manic depression."
Stunned
He told the court a psychiatric nurse who dealt with Samuel while he was on remand in Liverpool said the defendant told him he drowned Aidan in the bath and when his sister became distressed he did the same to her.
Mr Riordan said experts believed that when Samuel underwent his next episode of manic depression, he was likely to "unburden himself but not before then".
A "serious case review" involving all the authorities and agencies involved in the case, and with the family over recent times, has begun, with the result expected later in 2007.
But whatever the findings, the fact remains that only Samuel holds all the answers to what happened the evening he took it upon himself to end the short lives of his two children.
The killings had an impact not just on the children's mother and grandparents, but also stunned their home town.
The town's mayor, Doreen Jones, said: "They were children you always felt you wanted to give a hug to, because they were so loveable. And I know they were loved by all the family."