Human organs stored at South Tyneside District Hospital could date back 20 years.
Tissue samples were discovered at the hospital, in Harton Lane, South Shields, during an audit in March 2015.
Northumbria Police are working with colleagues in Cleveland to identify the samples and officers are visiting next of kin personally to explain the situation.
It is understood the samples were taken to the hospital by a pathologist who covered a large area of the North East over a number of years.
Police forces nationwide began carrying out audits in 2010 after it became apparent human tissue samples going back many years may have been retained.
All cases pre-date 2006, when the Human Tissue Act 2004 came into force.
Northumbria Police worked with the South Tyneside NHS Foundation Trust, local Coroners, the Human Tissue Authority and the Home Office to carry out an audit which was completed in March 2015 and police were made aware that some human tissue samples had been identified at South Tyneside.
The force informed Cleveland last year that a number of human tissue samples had been identified, related to cases from across the North East.
Specially trained Cleveland Police officers are in the process of visiting the 13 families involved in their force area and will be working with them to make sure that their wishes are carried out regarding their loved ones.
A spokesperson for Cleveland Police said: "We knew that this news would come as a shock to the families involved and so we decided that it was only right that specially trained officers visited them to give them the news in person.
"We will continue to work with Northumbria Police, the hospital, the coroner and the families to make sure that there is dignity afforded to the dead."
The Human Tissue Authority (HTA) said: “The HTA is aware of a situation in the North East of England where a number of items of human tissue, retained for criminal justice and coronial purposes over the course of the last 20 years, have been kept for longer than was necessary.
"We understand that an audit has been conducted by the authorities involved, and specially trained officers will be talking to all of the families affected.
"Discovering that the samples are still in storage will clearly be upsetting for these families, and of concern to the wider public; it is important to stress that significant improvements have been made to the systems governing the retention of human tissue samples and the public can be assured that the risks of this happening again are low.
"In any unexplained death, post-mortem tissue samples are taken as part of the investigation for evidential purposes and to find out the cause of death.
"In some cases, these samples have to be kept for significant periods of time to support the criminal investigation and fulfil legal requirements.
"The HTA has worked with the Home Office to develop police procedures on the management of human tissue, and there are clear standards and guidelines on the disposal of human tissue which HTA-licensed establishments, where these samples may be held, are expected to meet; you can read more about these in our updated Code on post-mortem examination, which is due to come into force in April this year: Draft Code of Practice B – Post-mortem Examination.
"We will be keeping in close contact with those involved in this case to provide any advice we can."
Detective Chief Inspector Lisa Theaker who is leading the operation said: “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.
"Victims and their families are our absolute priority and we will ensure their concerns are listened to and we will fully support their needs.
"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."
She continued: "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."