A prisoner has been arrested on suspicion of murder over an attack on a fellow inmate in one of the UK's safest prisons.
Robert Coello, from Berkshire, serving life for raping a child, died on Sunday after he was discovered in his cell at Grendon Prison and taken to hospital.
The BBC understands that he suffered head injuries after being stamped on.
Thames Valley Police arrested a 25-year-old man who has since been bailed back to the prison.
Coello, 44, from Whitley, was found by staff in his cell in the prison's G wing at 1540 BST on Sunday. He was taken to Stoke Mandeville hospital where doctors worked for seven hours to save his life, but he was pronounced dead just after 2300 BST.
A post-mortem examination is expected take place later.
Coello was jailed in 2006 after admitting four counts of rape and other offences, and told he must serve a minimum of seven years.
He later volunteered to go to HMP Grendon, a specialist category B prison which can hold 235 inmates. Each inmate is interviewed before admission and has to prove that they have a "genuine desire" to change, including staying off drugs.
In response to the death, the Prison Officers' Association said that recent budget cuts had put the prison under pressure.
But a Ministry of Justice spokesman said: "There has been no change to the type of prisoner held there in recent months, and no change to the vetting process."
If a prisoner is identified as a potential candidate for the treatment available at Grendon, staff assemble probation and psychiatric reports and intelligence from the prison service's offender assessment system. The prison's head of psychology decides which prisoners are appropriate to be admitted.
Once admitted, inmates are subject to three months of initial assessments before being allocated to one of five wings, each of which operates as an autonomous "therapeutic community".
This includes specialist education classes and allowing some prisoners to elect peers to leading roles in therapy sessions, and meetings with staff over some of the institution's workings.
Prisoners have separate cells and, according to inspection reports, the few double cells are never shared.
Last year, the then former chief inspector of prisons Dame Anne Owers praised Grendon as a "fundamentally safe place" with proven results in dealing with some of the most disturbed inmates in custody.
But she warned that its "remarkable achievements" could be put at risk by budget cuts.
"While the prison continued to provide a supportive and respectful environment for therapy, staff shortages and financial cuts had begun to impact seriously on therapy with cancellations of groups, reduced supervision and backlogs of therapy reports," she wrote.
"Further cuts had been proposed … [and] this exercise appeared to take little notice of Grendon's unique role."