Killer nurse Anne Grigg-Booth was hooked on the drug she used to poison her victims.

The former night matron at Airedale General Hospital, who was found dead at her home in Nelson, Lancashire, on Bank Holiday Monday, apparently fuelled her addiction to diamorphine, or heroin, by stealing the drug from the Steeton hospital's medical supplies.

Yesterday an inquest in Burnley confirmed a pathologist could not determine the exact cause of Grigg-Booth's death, but police said it was not being treated as suspicious. Samples have been sent away for toxicology tests.

Grigg-Booth was due to go on trial in April for the murders of three elderly women patients, the attempted murder of a 42-year-old man and 13 counts of unlawfully administering poison to 12 other patients.

Yesterday her solicitor Paul Fitzpatrick said he would be making a detailed statement on her behalf at Bradford Crown Court, at a hearing to prove her death and officially close the trial

Police investigating Grigg-Booth's actions focused on the senior nurse's injecting of painkilling opiates, including diamorphine, to patients between 2000 and 2002. The same drug was used by serial killer GP Harold Shipman.

During their investigation they suspected the 52-year-old had been a user of the class A drug, though she strongly denied this when questioned.

Strict rules surround the administering of diamorphine at Airedale. It is kept in a double locked cupboard. Two nurses must be present when it is taken from the unit and the numbers counted and checked against a drug control book.

According to Airedale NHS Trust's medicines code the administering of controlled drugs must involve two people - one to carry out the injection and the other to witness it.

Intravenous (IV) drugs like diamorphine should only be administered by doctors or authorised practitioners. The trust would not confirm if Grigg-Booth was considered an authorised practitioner trained in giving IV opiates.

Any trial into the murders ended with Grigg-Booth's death, but relatives of her victims are demanding answers. There have been three separate calls for inquiries into how Grigg-Booth managed to escape detection for two years.

The relatives called for a public inquiry, West Yorkshire Strategic Health Authority (WYSHA), in conjunction with Airedale NHS Trust, launched its own independent inquiry, and Bradford coroner Roger Whittaker plans to apply to the Lord Chancellor to conduct inquests into the victims' deaths.

Det Supt Phil Sedgwick, who led the investigation, said no-one would ever know how many patients Grigg-Booth had killed.

He said the Crown Prosecution Service (CPS) had decided to proceed with just three murder charges because cremations of other ill and elderly patients meant it was difficult to prove exactly how many died as a result of the nurse's interference.

When asked if he thought she may have killed more than three, he said: "She could have. We just don't know. Now we'll never know for sure. It's a very unsatisfactory ending."

Grigg-Booth, formerly of Farling Top, Cowling, was arrested in March 2003 after hospital bosses raised concerns about the death of 96-year-old Annie Midgley, whose records were examined during a routine audit of deaths at the hospital. This sparked a police investigation.

Divorcee Grigg-Booth was finally charged in September 2004. She made her first appearance at Bradford Crown Court in October, when she was remanded on conditional bail pending trial.

In recent months Grigg-Booth appeared to have become depressed. Neighbours said her health had declined and she was drinking heavily.

Next-door neighbour Michael Howarth said she had even been treated at Burnley General Hospital the day before her death after a fall.

Grigg-Booth, who had a son, had always denied being responsible for any deaths and given no clues to a possible motive.

Special report page 2