A rise in the number of killings in prison lays bare the "unacceptable" levels of violence behind bars in England and Wales, a watchdog has warned.
Official figures show there were six apparent homicides in jails in 2015-16 - the largest number for a year to March since current records started in 2000.
Prisons and Probation Ombudsman Nigel Newcomen examined eight cases that have been investigated since the beginning of 2013 in a new report.
He said homicides are still rare in prisons, although they have increased over recent years.
"Yet the killing of one prisoner by another in a supposedly secure prison environment is particularly shocking, and it is essential to seek out any lessons that might prevent these chilling occurrences in future," Mr Newcomen said.
He added: "The cases we studied had little in common beyond their tragic outcome.
"Nevertheless, what is clear is that the increased number of homicides is emblematic of the wholly unacceptable level of violence in our prisons."
The eight fatalities studied occurred at different establishments, from local to medium and high security prisons.
Some involved weapons, others strangulation or, in two cases, a punch.
The outcome of the criminal proceedings varied. In four of the eight cases, one or more prisoners were convicted of murder, three cases led to manslaughter convictions, and in one case the prisoners concerned were acquitted.
The PPO found that too little consideration was given to events that might have made the victim vulnerable to attack in one case in the sample.
In two others it was found that it would have been difficult for the prison to have identified that the victim was at particular risk from their attacker, but there were concerns over "the lack of a structured and co-ordinated approach to challenging violent behaviour".
The ombudsman concluded it would have been difficult for staff to have predicted or prevented the death in the five remaining investigations.
Mr Newcomen said there was a need to better manage violence and debt in prison - including that associated with the "epidemic" of new psychoactive substances - formerly known as legal highs.
The report also raised concern that police were not called until more than three hours after an assault in one incident.
Other "areas of learning" include the need for rigorous cell searching to minimise the availability of weapons.
Concerns have repeatedly been raised about safety in jails in recent months. In the year to the end of March there were 5,423 assaults on officers - up 40% on the previous year.
Earlier this year Peter Clarke, the chief inspector of prisons, warned that too many prisons have become "unacceptably violent and dangerous places".