Jennifer Gibbons, 29, and her twin, June, were sent to Broadmoor when they were 18 after carrying out a spate of arson attacks. From the age of about three they would talk to almost no one but each other.
The pair - recently diagnosed as schizophrenic - were taken to a clinic in Bridgend, south Wales, on Tuesday as the first step towards rehabilitation. As soon as Jennifer arrived medical staff noticed that she was very ill. She was taken to hospital but died two hours later. The results of a post- mortem were inconclusive; further tests are being carried out.
The twins' condition has baffled psychiatrists, who have described them as 'elective mutes'. At school they refused to read or write and gradually excluded everyone - including their younger sister, Rosie, and their parents - from their silent world.
However, they were both prolific writers. After their arrest for arson police discovered two sacks of diaries, poems, short stories and novels hidden at their home in Haverfordwest, Dyfed.
In Broadmoor, near Reading, they kept diaries written in tiny script. Their writing revealed a love-hate relationship between them. While on remand, June wrote about Jennifer: 'I really aim to be alone. Yet, I am deceiving myself. Can I stand being alone? My heart does not beat so fast now. It only beats fast when J is around.'
They were kept in separate wards while at Broadmoor and on several occasions attempted suicide and attacked one another. Despite this they spent much of their social time together. Recently, they had begun to speak more openly to hospital staff, other patients, their family, and Majorie Wallace, a journalist.
Ms Wallace, now chief executive of the schizophrenia charity, Sane, visited the pair on 28 February and noticed that Jennifer had lost a lot of weight. She said: 'She was very thin, but was talking a lot and giggling. She said she was eating properly and would feel better when she left the hospital.'
The twins were taken to Caswell Clinic interim secure unit, in Glanrhyd Hospital, Bridgend, on Tuesday morning. Dr Chris Hunter, the unit's clinical director, said: 'As soon as she (Jennifer) got out of the van it was noted she was physically unwell.'
Jennifer was immediately taken to the Princess of Wales Hospital, Bridgend, but died at 6.30pm before tests could establish what was wrong with her.
A spokeswoman for Broadmoor hospital said an internal inquiry into the health care she had received during the past two years would be carried out.
Michael Morgan, director of administration at Broadmoor, refused to comment on Jennifer's medical condition, except to say: 'If she had been unwell she would not have been transferred.'