Police are contacting the relatives of deceased patients after it was discovered that organs have been stored at a hospital for "longer than necessary".
Human tissue samples were discovered at South Tyneside District Hospital, in South Shields, during an audit in March 2015.
Northumbria Police and Cleveland Police are now trying to identify the samples and said officers are visiting next of kin personally to explain the situation.
A Northumbria Police spokeswoman said: "The audit was completed in March 2015 and we were made aware that some human tissue samples had been identified at South Tyneside District Hospital that have been kept longer than necessary.
"We have been working with Cleveland Police to establish the identity of the samples and to find the relevant next of kin."
The audit was carried out jointly between Northumbria Police, South Tyneside NHS Foundation Trust, local coroners, the Human Tissue Authority and the Home Office.
Detective Chief Inspector Lisa Theaker, who is leading the operation, added: "We understand this will be upsetting to those involved and we will offer them as much support as possible.
"We have specially trained officers who are personally visiting each family to explain the situation.
"We have made significant improvements in relation to the management and retention of human tissue samples and we want to reassure the public that risks of this happening again are low.
"In any unexplained death, human tissue samples are retained as part of the investigation for evidential purposes and to determine the cause of death.
"In some cases, this material can be retained for significant periods of time to support the criminal investigation and fulfil legal requirements.
"We will continue to fully support the families through this process and ensure their wishes are met."
Sarah Simpson, from Blakelaw, Newcastle, told the Sunday Sun newspaper that her father's organs had been taken without his family's knowledge and stored at the hospital for 22 years.
She said her father, Terry, died after falling from a block of flats in Newcastle in April 1995 and his family cremated him believing his body was intact.