One of Scotland's most notorious ­prisoners has been found dead at a high security hospital more than three years after killing her three children amid a custody dispute.

Police confirmed that they had been called around 2am yesterday to Rampton Hospital near Retford, Nottinghamshire, after the body of American-born Theresa Riggi, 50, was discovered.

A police spokesman said the death was "unexplained but non-suspicious".

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Rampton is one of three high security hospitals in England and treats the criminally insane. Soham killer Ian Huntley and "Angel of Death" nurse Beverly Allitt are among its past and present patients.

California-born Riggi was transferred there last year from a psychiatric unit in Scotland. She was previously a prisoner at Cornton Vale women's prison near Stirling.

The triple child-killer had become a target for other inmates and was reportedly disfigured for life following an attack in November 2011 by a fellow prisoner.

Riggi stabbed her children - eight-year-old twins Austin and Gianluca, and Cecilia, five - eight times at their second-floor flat in Edinburgh on August 4, 2010. She then set off a gas explosion in a bid to cover up the killings before leaping from the balcony in an apparent suicide attempt.

She survived her injuries and was charged with their murders. She later pled guilty on a reduced charge of culpable homicide on the grounds of diminished responsibility. Jailing her for 16 years in 2011, Judge Lord Bracadale told the High Court in Glasgow the killings were "truly disturbing" and "grotesque".

Before sentencing, Riggi was diagnosed with three types of personality disorder: narcissistic, paranoid and histrionic.

She had moved to Edinburgh with her children from Skene, Aberdeenshire, following the break-up of her marriage to the children's father, oil industry executive Pasquale Riggi.

Mr Riggi, originally from Colorado, described the loss of his children as an "overwhelming tragedy".

Riggi and her husband were involved in a legal action at the Court of Session in Edinburgh over the children at the time of the tragedy.

Riggi was said to be extremely protective of her children, teaching them at home and not allowing them much contact with the outside world. When they went to meet their father, Riggi made them wear tags and gave them all mobile phones so they could remain in contact with her.

In July 2010, Riggi sparked a missing persons inquiry when she disappeared with the three children. Police traced them to a flat on the west side of ­Edinburgh later that month, but ­officers took no further action.

Riggi failed to turn up to a custody hearing the following month, prompting the judge to rule that she be tracked down and a decision taken by social workers as to whether her children should be taken into care.

The next day she killed them and attempted to take her own life.

Rampton houses 326 patients. Many have committed extremely violent crimes while others are admitted for their own protection.

In 2011, chief executive Dr Mike Harris dismissed claims the hospital was a "holiday camp" despite criticisms of the unit's extensive grounds, swimming pool and full Sky Sports TV package.

He added: "My number one priority is to safeguard the public but if this was my child in this hospital, I'd want them to receive the very best treatment and appropriate accommodation."

A Nottinghamshire Police spokesman said: "We were called to Rampton in the early hours of [Monday] morning to the death of a woman.

"It's not being treated as suspicious and we are preparing a file for the coroner."

A Scottish Government spokesman said: "We are aware that a Scottish patient has died at Rampton Hospital. It would be inappropriate to comment further in light of on-going investigations."

A file on Riggi's death being prepared for the coroner and an inquest will be held.