Three men, arrested on suspicion of murder after an inmate died in prison, have been released under investigation.
Thames Valley Police said a 49-year-old man died after an "altercation" at HMP Woodhill in Milton Keynes on Tuesday.
It is understood the arrested men, aged 32, 33 and 26, who were also inmates, have been returned to prison.
Thames Valley released the men from police custody while inquiries continue.
The authorities have not publicly released the name of the dead man.
Woodhill is a Category A jail which has about 800 prisoners.