The Police Ombudsman's Office in Northern Ireland has revealed that it has also stored body parts of victims of violent deaths without informing family members.
The revelation that the independent police complaints investigation team holds human tissue from four people comes after the Police Service of Northern Ireland (PSNI) announced that it retained parts of 64 crime victims.
A spokesman for the Ombudsman's Office apologised to the families of the deceased for not telling them that samples were taken from their loved ones.
The PSNI's disclosure was prompted by a UK-wide audit of retained tissue by police forces
A spokesman from the Ombudsman's Office said it carried out its own audit on the back of that move.
He said: "We have completed this work and have established that we hold items of human tissue from the bodies of four people. The people in question died in incidents during the period from 2001 to 2006, all of which have been subject to Police Ombudsman investigation.
"The families of these people will have been contacted by the end of the week. We will provide them with more specific details and apologise in full for not having provided them with this information much earlier."
On Wednesday, the PSNI also apologised for failing relatives. Around a third of the 64 crime victims involved were Troubles-related deaths in Northern Ireland. Most were murdered.
Investigators kept human tissue and body parts as evidence from 1960 to 2005, with some held for substantial periods. The law at the time did not compel them to seek or secure the consent of families.
In 2010, the Human Tissue Authority (HTA) issued a direction asking all mortuaries holding post-mortem tissue samples to undertake an audit and report back to the authority and all chief constables in England, Wales and Northern Ireland were asked to conduct a review.