A prisoner killed at HMP Belmarsh has been named as convicted arsonist Sundeep Ghuman.
The Metropolitan Police said officers were called to reports of an assault at the south east London prison at 8pm on February 18.
Ghuman, 36, was found by officers suffering from head injuries and was taken to hospital in a critical condition.
He died in hospital the following day.
Two male inmates - both aged 28 - were arrested at the scene on suspicion of attempted murder, and have sinced been bailed.
A post-mortem examination of Ghuman, which took place at Greenwich Mortuary on February 21, gave the cause of death as a head injury. Police said his next of kin have been notified.
Ghuman was jailed for five years and four months in 2011 for a series of arson attacks in the south east London borough of Bexley, the Crown Prosecution Service confirmed.
The Bexley Times reported at the time that Ghuman broke into a Kwik Fit store on May 7 2011 while armed with a blow torch and a propane gas canister, and set fire to a tyre stack and a fence.
Later that night, residents had to be evacuated from their homes after he damaged a pipe, causing a build-up of gas.
He was found guilty of two counts of arson and three counts of being reckless as to whether life would be endangered, the paper reported.
At the end of his prison sentence, Ghuman was to remain on licence for a further five years.
Detective Chief Inspector Richard Leonard from the Metropolitan Police said: “We are working to establish the circumstances surrounding this incident with the support of the prison and local officers.
“At this stage I am open minded concerning motive, but expect this to become clear as the investigation progresses.”
Belmarsh is one of only three high-security “core” local prisons in England and Wales and has housed some of Britain’s most high-profile prisoners, including Michael Adebolajo, one of the men who killed Lee Rigby.
Finsbury Park terrorist Darren Osborne and radical preacher Anjem Choudary were also jailed there.