A Dutch nurse accused of murdering 13 patients, including four babies and a UN war crimes judge has proclaimed her innocence at the start of her trial in The Hague.

Lucy de Berk allegedly injected each of the victims with a lethal cocktail of drugs. The killings are said to have taken place at three hospitals in The Hague, where she worked over a four-year period.

I have a clear conscience. I didn't do a thing. Of course it's strange, but I don't know how it happened

Lucy de Berk

The authorities became suspicious last September when a five-month-old baby in her care died unexpectedly just after Ms De Berk finished duty.

Denying all responsibility, Ms De Berk instead blamed doctors for ignoring warnings that children in her care were dangerously ill.

The case, which is being heard by three judges in front of a packed public gallery, has sparked immense media scrutiny in the Netherlands.

Mystery

In court, Ms De Berk gave her account of the death of a six-year-old Afghan boy, Ahmad Noory, who died during her shift at the Juliana Children's Hospital.

"I warned the doctor that the child was very ill and nothing was done. Nobody did anything when I told them Ahmad had stopped responding and couldn't be woken up," she said.

The case has aroused massive media interest

When asked to explain why all of the deaths occurred when she was on duty Ms De Berk said: "I have a clear conscience. I didn't do a thing. Of course it's strange, but I don't know how it happened."

This was the first time that the nurse had given up her right to silence and answered prosecutors' questions on the subject.

The nurse, who also faces five charges of attempted murder and is accused of forging her high school diploma to qualify for medical training, has refused psychological assessment.

Deadly drugs

Prosecutors have described 40-year-old Ms De Berk as a classic psychopath who is "obsessed with death".

The nurse allegedly injected her victims with small but deadly doses of potassium and morphine at three hospitals where she worked from February 1997 to September of last year.

Suspicions were sparked when a five-month-old baby died in September 2001, just an hour after doctors had said its health was improving.

A post mortem examination on the child indicated foul play and prompted an investigation.

In March, investigators exhumed the bodies of three infants initially believed to have died of terminal illnesses. Traces of poisonous toxins were found in their blood.

Weight of evidence

A number of her alleged victims were children born with serious physical abnormalities and several elderly or terminally ill patients, who were initially believed to have died from natural causes.

One of them was a 91-year-old Chinese judge at the Yugoslav war crimes tribunal, Haopei Li, who died in 1997, just weeks before he was due to retire.

Among those who will be giving evidence at the trial are a professional serial killer profiler from the US Federal Bureau of Investigation and a mathematician.

The mathematician specialises in calculating probabilities and will be called upon to demonstrate how likely it is that the same nurse could be on duty when all of the deaths occur, purely by coincidence.