Four people have been arrested on suspicion of assault and misconduct in public office at a prison where a series of inmates have died in the last year.
South Wales Police arrested the people, who are all staff at HMP Parc in Bridgend, on Friday, following a series of incidents at the prison.
The police have not said what the incidents relate to, but 13 inmates are known to have died at HMP Parc this year.
Detective Chief Inspector Dean Taylor said: “We received concerns about the conduct of staff at HMP Parc on Wednesday September 18.
“Officers are at the early stages of the investigation and are working closely with G4S while inquiries are ongoing.”
The four arrested are a 23-year-old woman from Bridgend; a 45-year-old man from Pontycymer; a 25-year-old man from Merthyr and a 35-year-old man from Llanelli.