Police have named a prisoner who was stabbed to death at HMP Wormwood Scrubs.
Saleh Khader was found with a number of knife injuries on Wednesday afternoon and was pronounced dead at the scene.
Four inmates at the west London jail have been arrested on suspicion of murder.
The men, two aged 21, a 20-year-old and a 23-year-old, remain in police custody.
In a statement Scotland Yard said: "While formal identification awaits, officers are confident the deceased is 25-year-old Saleh Khader.
"A post-mortem examination will be arranged in due course."
The victim's next of kin have been told, police added.