A second girl has died at a mental health unit for young people where staff face being disciplined for allegedly mistreating patients.
Admissions to Middlesbrough's West Lane Hospital were temporarily suspended after 17-year-old Christie Harnett, from Newton Aycliffe, died in June.
Now it has emerged another girl, also 17, died on 9 August in circumstances yet to be revealed.
Police said they were investigating the latest death.
In March it was confirmed 13 members of staff were facing disciplinary action over alleged ill treatment of patients.
Colin Martin, chief executive at Tees, Esk and Wear Valleys NHS Foundation Trust, said: "Tragically a 17-year-old who was a patient at West Lane Hospital died on Friday.
"We take the recent concerns relating to West Lane Hospital extremely seriously and we are committed to making the improvements required to ensure that we provide safe, high quality care.
"In June, concerns were raised by the Care Quality Commission around recording of observations, risk assessments and staffing levels given the acuity of patients.
"As a result we have instigated a significant programme of work.
"Doing this helped us to address staffing and safety in order to better meet the needs of the young people in our care."
Cleveland Police said in a statement: "Police were made aware of the tragic death of a female, aged 17, on 9 August.
"The female was an inpatient of West Lane Hospital. but passed away in James Cook University Hospital in Middlesbrough.
"Officers are conducting inquiries into the circumstances surrounding her death."
Middlesbrough's Labour MP Andy McDonald said deaths at the hospital raised "serious questions".
He added: "Clearly a number of formal inquiries are now under way, but the authorities do need to spell out publicly as a matter of supreme urgency what additional measures are being put in place now to safeguard the patients at West Lane."
A CQC spokesperson said: "In response to concerns raised with us in June, we conducted a responsive unannounced inspection.
"Inspectors returned to the hospital last week in response to further concerns and are currently compiling a report.
"The concerns have been made very clear to the trust and consideration is being given to further enforcement action."