A 22-year-old Helston woman was arrested this week in connection with a probe into neglect at a Penzance care home.
The John Daniel Centre in Penzance caters for more than 80 people with learning disabilities.
A number of staff were suspended in December after concerns were first raised.
Following further enquiries by police and other partner agencies a 21-year-old man from St Ives, a 58-year-old man from Penzance and a Hayle man and woman, both aged 57, were also arrested on suspicion of causing wilful neglect.
The investigation continues.
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