Three prisoners have been arrested after another inmate was found dead in a cell.
A murder investigation was launched after the 23-year-old man, originally from Wolverhampton, was discovered by prison staff at HM Prison Frankland just after 10am on Saturday, Durham Police said.
Three men were arrested at the scene and will be questioned by detectives.
A Durham Police spokeswoman said: "Police are investigating the death of a man in HM Prison Frankland, Durham.
"Prison staff were alerted just after 10am and found a seriously injured prisoner in a cell. He was pronounced dead at the scene.
"A homicide investigation is under way."
The spokeswoman continued: "Three men, who were also prisoners, were arrested at the scene and arrangements are being made for detectives to interview them.
"The cell where the man was found has been cordoned off pending a full forensic examination."
The spokeswoman said the man would not be named until his family had been informed of his death.
A post-mortem examination will be carried out in due course, she added.