Ten workers have been arrested over the alleged abuse of patients with learning difficulties at a specialist hospital.
Seven men and three women were arrested at addresses in Barnard Castle, Bishop Auckland, Darlington and Stockton.
Undercover filming by BBC Panorama at Whorlton Hall in County Durham appeared to show patients being mocked, intimidated and restrained.
The site had at least 100 visits by official agencies in the year before the abuse was discovered.
Those arrested were being questioned about offences relating to abuse and neglect at the privately-run NHS-funded unit, Durham Police said.
A spokesman said investigations were expected to take some time but repeated the force's "immediate priority has been to work with other agencies to safeguard the victims at the centre of the allegations and their families".
Those arrested would be released under investigation pending further inquiries, he added.
The force said it was seeking the co-operation of the Panorama team to gather further evidence.
Cygnet, the firm that runs the 17-bed hospital unit for adults with learning difficulties and autism, said it was "shocked and deeply saddened" by the allegations.
The company only took over the running of the centre at the turn of the year and said it was "co-operating fully" with the police investigation.
All the patients have been transferred to other services and the hospital closed down, Cygnet said.
Care minister Caroline Dinenage told the House of Commons this week she was "deeply sorry that this has happened".