Police are investigating almost 30 years of alleged physical and sexual abuse against children and young adults being treated at a mental health unit.
Hertfordshire Constabulary said the number of victims at the Hill End Hospital Adolescent Unit in St Albans was not yet clear.
Investigators said a criminal investigation had started into “reports of physical and sexual abuse against patients between 1969 and 1995, when the facility closed”.
They are appealing for anyone with information that could help the probe, codenamed Operation Meadow, to come forward.
Assistant Chief Constable Bill Jephson said: “Hertfordshire Constabulary takes all reports of child abuse seriously, no matter how long ago the offence may have occurred.
“We have officers who are specially trained to deal with these type of crimes while supporting victims in a sensitive, empathetic and caring manner.”
The force is bracing for an influx of reports, and has readied specialist officers alongside NHS services and charities to support victims and witnesses coming forward.
It comes after sexual abuse was uncovered at several hospitals and local authority care units.
Jimmy Savile abused child patients at Stoke Mandeville Hospital over two decades, while late politician Cyril Smith targeted children at a school, hostel and other institutions in Rochdale.
Two men who allegedly assaulted children while dressed as Father Christmas are among dozens of abusers who targeted girls and boys at the Shirley Oaks children’s home until it closed in the 1980s.
Current and former cases are being investigated by the ongoing Independent Inquiry into Child Sexual Abuse.
It is taking evidence on “the extent to which institutions have failed to protect children from sexual abuse” in councils, the police, armed forces, schools, hospitals, children’s homes, charities, religious groups and other public services.