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Nurse and mother Kate Bagby was at work caring for patients the day she heard her son had been murdered.

"I will never forget that phone conversation as long as I live," says the former nurse, 66, who is originally from Rochester, Kent.

"It was a policeman and he was telling me: 'Your son is dead under suspicious circumstances.' "All I could say was: 'My son is dead?'"

Then Kate collapsed.

"The murder has dominated our lives," says her husband, David, 63. "You ask yourself, 'How could that happen?'"

It soon became apparent who was responsible. Shirley Turner, a 40-year-old mother of three and Andrew's on-off girlfriend, had fled a bloodstained scene littered with incriminating evidence.

But the Bagbys, who live in San Jose, California, could scarcely imagine what lay in store for them. Turner had not finished with the family yet.

Andrew Bagby was an easygoing man with a wide circle of friends who found it almost impossible to say no to people.

This attracted the needy and insecure Turner to him. She worked as a doctor at the Memorial University of Newfoundland, in Canada, where Andrew trained. Twelve years older, she became obsessed with him. "She was difficult to get to know," says Kate. "She was not the woman you wanted involved with your son. I didn't like her but I didn't think she was capable of murder."

As the couple continued to date, Andrew complained to friends that Turner would not leave him alone. Once, she called him 30 times while he was at a pal's wedding.

By this stage, he had moved to a new job in the US. On November 3, 2001, he broke up with Turner at the airport before putting her on a plane back to Council Bluffs, 1,300 miles away.

Two days later, at 5.30am, Turner turned up at his Penntment, sylvania apartment, having driven 16 hours to see him.

Colleagues warned him to go to the police but, instead, after work that day he drove to a car park to meet her.

The next day, his body was found riddled with bullets. He had been shot five times - in the face, chest, twice in the buttocks and once in the back of the head.

Spent casings matched Turner's handgun and a witness saw a car similar to hers parked next to Andrew's car.

It looked like an open-and-shut case but, before the police could arrest her, Turner fled over the Canadian border, back home to St John's, in Newfoundland.

More than a month passed before Turner was finally arrested in Canada on December 21 but, within hours she was out on bail until she could be extradited to the US.

Two months later, with the extradition process still grinding on, Turner announced she was pregnant with Andrew's child.

At first, the Bagbys did not know whether to believe her.

But if her claims were true, and she was carrying their grandchild, then they were clear: they wanted custody.

Deadly trade-off

They moved to Newfoundland to give the legal battle their full attention but Turner remained at large. In July 2002 she gave birth to a boy.

"The birth of Zachary gave us part of Andrew back," says Kate.

"We never expected it. When we met him, we just melted as if it was magic. He was beautiful and looked just like Andrew.

"It was an overpowering feeling. He was in a little crib and David and I knelt down at his feet and he was just, beautiful."

Shortly after Andrew's death, his friend Kurt Kuenne had started to a make a documentary about the murder, asking friends and family to share their memories of Andrew.

Zachary's birth gave the documentary new impetus. Now called Dear Zachary, it would be a "letter" to Andrew's son about the father he would never meet.

Meanwhile, Kate and David's bond with Zachary grew stronger. He was a happy baby, with strawberry-blond hair and Andrew's nose. They smothered him in kisses and often spoke to him about Andrew.

Turner, despite being under orders to avoid contact with the couple, sent them photographs of the baby and a "baby book" so they could fill out their side of a family tree.

The couple were so scared of upsetting Turner and losing access to Zachary they forced themselves to be cordial.

"The law had let her out on the street and let her keep formal custody of the baby," says David. "We had to deal with her to get to Zachary."

In November 2002, a year after Andrew's murder, the judge in Turner's extradition case finally threw out the queries raised by her lawyer and put Turner in jail, ruling there was enough evidence to warrant a trial.

Suddenly, the Bagbys were appointed Zachary's main carers. It was the result the couple had been waiting for.

But in January 2003, the legal system confounded their expectations yet again. A judge ruled that Turner's detention was not in the public interest and released her on bail.

Finger on the trigger

For the second time, the Bagbys were left stunned at the leniency of the courts. They were forced to hand Zachary back to Turner and, again, tiptoe around her.

"Every time I saw Shirley I'd think, 'That's the finger that pulled the trigger on the gun that killed Andrew,'" says Kate.

"It was like being at war," adds David. "Nobody wants to do it, but you have to do it."

The "war" would last six months.

In June 2003 the Attorney General ordered that Turner be extradited to the US. She appealed and a hearing was set for September 2003.

Then, on August 16, six weeks before Turner was to appear in court, the Bagbys went with her and Zachary to a swimming pool.

Zachary enjoyed himself in the water and a couple of hours later the Bagbys drove Turner and the baby home.

By the time they arrived at Turner's house, Zachary was asleep and Kate and David prepared to leave. But Turner did a strange thing: she woke Zachary up and said to him: "Say goodbye to Grandma and Grandad."

He started to cry and reached out for Kate, but Turner held him back and said firmly: "You can't always be with your grandparents."

That was the last time the Bagbys saw Zachary alive. At 3am on Monday, August 18, Turner drove to a near by marina . With Zachary strapped to her stomach with a sweater, she jumped into the Atlantic Ocean.

Their bodies were later washed up on a beach. When the Bagbys visited the mortuary and saw Zachary's tiny body covered by a white sheet, they fell apart.

"We had a complete meltdown," says David. "It's a blur of emotion." The pain of having lost not only their son but now also their grandson was too much to bear.

The couple left Canada and returned to California - but the tortuous memories followed.

David has written a book about the case, Dance with the Devil: A Memoir of Murder and Loss.

He is campaigning to change Canadian bail law so no one charged with murder is allowed to walk the streets like Shirley Turner.

"We had to do something to channel our rage and sorrow," he says. Kurt's film, meanwhile, was released last year and hailed in the US as one of 2008's most moving documentaries.

"When people have seen Dear Zachary, they gasp," says David. "They are appalled and outraged."

On the film's website, David asks those who have seen Kurt's documentary to write to the Canadian Minister of Justice and add their name to his campaign.

So far, nothing has changed - but continuing the legal battle helps give David and Kate the strength to carry on.

"In a small sense we have found some peace and contentment," says David.

"Kate and I are very close and that helps. Part of my peace comes from doing everything to change Canadian law."