HSSIB is fully independent arm’s length body of the Department of Health and Social Care. It is independent of the NHS, it is hosted under the Care Quality Commission (CQC). They can investigate patient safety concerns that:
occur in England during the provision of healthcare services; and
have or may have implications for the safety of patients.
Health Services Safety Investigations Body (HSSIB) investigations can consider healthcare provided in the NHS and the independent sector where safety learning could also help to improve NHS care
For more information see here.