A Dutch court has delayed its verdict on a nurse accused of killing 13 patients, ordering her to undergo psychiatric examination and demanding that the original murder inquiry be reopened.

The judges also said they want more information from some witnesses.

The court is re-opening the investigation. The information we had is incomplete

Presiding judge Jeanne Kalk

Lucy de Berk, who denies responsibility, allegedly injected each of the victims with a lethal cocktail of drugs. The dead include four babies and a UN war crimes judge.

She had previously refused prosecution requests for psychiatric tests, and the court had not ordered them to go ahead.

At Tuesday's hearing, the three judges said forensic information offered to the court was inadequate, and they wanted a further examination on possible sources of chemicals found in the bloodstream of some of the victims.

The judges also wanted to question some witnesses who gave testimony in depositions and did not appear in court.

"The court is reopening the investigation," said presiding judge Jeanne Kalk. "The information we had is incomplete."

The case will be reviewed within three months, but no new date was set for the trial's resumption.

If convicted, Ms De Berk would be the worst serial killer in Dutch criminal history and faces life in prison or in a mental health facility.

I took pleasure in my work. No-one told me once that I didn't do my work well. Never

Lucy de Berk

The case has generated enormous media interest in the Netherlands, the first country in the world to have legalised euthanasia, albeit under strict guidelines.

Ms De Berk is alleged to have injected her victims with small, but deadly doses of potassium and morphine at three hospitals in The Hague where she worked from February 1997 to September of last year.

During her five-day trial in September, prosecutors said the deaths that occurred on her shifts followed a pattern showing the same hand at work in each case.

But defence lawyer Ton Visser argued there was no evidence linking Ms De Berk directly to a single crime.

The case has aroused massive media interest

He said the prosecution's case was built on "circumstantial evidence and coincidence" and was not enough to convict his client of murder.

Among the prosecution witnesses were an FBI expert who testified that Ms De Berk fitted the profile of a serial killer, and a statistician who estimated that the odds of Ms De Berk happening to be on duty when all the deaths occurred were less than one in 300 million.

Ms De Berk has consistently protested her innocence.

"I took pleasure in my work," she said during the trial. "No-one told me once that I didn't do my work well. Never."

The nurse, who also faces five charges of attempted murder, admitted during the trial that she had lied about her nursing credentials and had stolen books, medicine and patient files from the hospitals where she worked.

Deadly drugs

Suspicions about the deaths 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.

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 international war crimes tribunal in The Hague, Haopei Li, who died in 1997, just weeks before he was due to retire.