Police are refusing to say how many families are involved in a tissue sample storage probe at South Tyneside District Hospital.
Some of the samples date back 20 years old and were found at the hospital, in Harton Lane, South Shields, during an audit two years ago.
Northumbria Police say they are contacting relatives and next of kin to explain how they came to be kept “longer than necessary” after being taken as evidence during investigations into deaths.
Cleveland Police has confirmed it is speaking to 13 families but Northumbria says it has decided not to make public how many people are involved in their area.
It is understood the samples were taken to the hospital by a pathologist who covered a large area of the North East.
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 the cases in South Tyneside pre-date 2006, when the Human Tissue Act 2004 came into force.
The audit was completed in March 2015 and police were made aware that some human tissue samples had been identified at South Tyneside District Hospital that have been kept longer than necessary.
Det Ch Insp Lisa Theaker, of Northumbria Police, 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.
“In any unexplained death, human tissue samples are retained 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.”
A spokesman for The Human Tissue Authority said: “The HTA is aware of a situation 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 will be keeping in close contact with those involved in this case to provide any advice we can.”
Cleveland Police are also part of the investigation by speaking to 13 families involved in their force area.
Terence Carney, HM senior coroner for Gateshead and South Tyneside did not want to comment.