Some families may learn for the first time that their relatives' deaths were linked to Army IRA agent Stakeknife, a senior police officer said.
Significant new evidence has been uncovered by the English police chief leading an independent investigation into more than 50 murders.
A probe headed by Bedfordshire Police Chief Constable Jon Boutcher into the high-ranking mole who led the republican organisation's "nutting squad" internal security unit while employed by the state is appealing for peripheral figures during offences to come forward.
He said: "We have reached a stage in the investigation now where we have spoken to almost all the families that we need to talk to.
"There are some families that we are still going to approach who we believe because of the inquiries that we conducted their loved ones and the murder of their loved ones will fall within our investigation.
"It will potentially be news to them."
The investigation is centred on possible crimes by paramilitaries, agents and Army and police handlers linked to Stakeknife, allegedly the military's highest-ranking spy within the IRA, and wants to bring people before the courts.
Multiple murders, attempted murders and unlawful imprisonments are included in the probe.
The chief constable added: "The further we progress into the investigation the more optimistic I am that we can achieve a successful outcome but only time will tell."
He said he was primarily pursuing the torturers and murderers and attempting to establish if members of the security forces were involved in action or inaction which led to these offences.
Mr Boutcher added: "This is not about those people who were on the periphery, who provided the premises or transport or guarded people.
"The investigation is focused on those responsible for the torture and murder of these victims."
Almost 50 detectives are working on the Stakeknife investigation.
A group of six international policing experts has been appointed to inform the investigation on a voluntary basis. They include senior police officers from the US, Scotland, an Australian ex-officer and ex-Northern Ireland police ombudsman Nuala O'Loan.
In 2003 Stakeknife was widely named as west Belfast man Freddie Scappaticci but he has always strongly denied the allegation.
The investigation was launched after Northern Ireland's Director of Public Prosecutions Barra McGrory referred the multiple allegations to the Police Service of Northern Ireland (PSNI).
PSNI Chief Constable George Hamilton asked external police to undertake the probe in an effort to ensure its independence. No former or current officers who have served in Northern Ireland will work on it, nor will ex or serving Ministry of Defence or Security Service personnel.
It is funded by the PSNI.
The investigation asked anyone who had any information to call the dedicated phone line on 01234 858298 or email firstname.lastname@example.org.