A prisoner who died inside a privately-run jail may have cut his own throat, the Met Police said.
The inmate, in his 40s, died after his throat was slashed at HMP Thameside in Greenwich, south-east London, at about 02:30 GMT on Sunday.
He was pronounced dead at the Category B prison run by the private company Serco, which holds some 1,200 inmates.
Police said a male prisoner in his late 30s arrested on suspicion of murder has been released, under investigation.
“Officers are working to establish the full circumstances, including whether the deceased’s injuries were self-inflicted or the result of an assault,” the Met Police said.
Earlier, officers said the inmate was discovered “suffering from a slash injury to his throat” and he was pronounced dead at the scene by paramedics.
“Officers are working to inform his next of kin. Formal identification and a post-mortem examination will take place in due course,” a spokesperson added.
Trish Mitchell, Serco contract director at HMP Thameside, said: “Sadly there has been a death at HMP Thameside. A man has been arrested and our staff are working closely with the police.”
HMP Thameside, which opened in 2012, serves courts in east and south-east London and has an “extremely high turnover of prisoners” with the average stay a “mere 36 days”, according to an inspection report in 2017.
HM Chief Inspector of Prisons Peter Clarke found the jail had a “generally settled and respectful atmosphere” but noted that “levels of violence were high” although not as high as “the huge rises in violence” at other jails.