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.
An HMP Parc spokesperson said: “The vast majority of our staff are hardworking and honest.
“We are absolutely committed to rooting out any wrongdoing.
“We take these allegations very seriously and are fully supporting the police with their investigation.”
In May, the former UK Conservative government faced calls to take over the running of the prison from private firm G4S, amid concerns over drug use and deaths at the prison.
G4S has managed the establishment since it opened in 1997 and it received a 10-year contract to continue operating it in 2022.
Former justice minister Edward Argar said at the time the contract “continues to perform well”.
The Welsh government’s social justice minister, Jane Hutt, told the Senedd on Wednesday that the situation at the prison was now “much more stable”.
She said she met Ian Barrow, the head of HM Prison and Probation Service Wales, in August, who reassured her “on the progress made at Parc” since the deaths in custody earlier this year.
Ms Hutt added that she would be visiting the prison later this month with Lord Timpson, the UK government’s prisons minister.