A specialist team of detectives has begun an investigation into more than 3,000 unsolved murders committed during the Troubles.
The Historical Enquiries Team began its work yesterday with a team of about 100 investigators and supporting staff, and a budget of £30 million (€44 million).
They will take up to seven years to try to uncover more about the 3,269 unsolved killings committed before the signing of the Belfast Agreement in 1998.
The PSNI hopes fresh investigations into the "cold cases" will help bring some form of resolution to the victims' families.
Chief Constable Sir Hugh Orde said: "The families have a right to know as much as we can tell them. If there are lines of inquiry to follow which gives us an opportunity to go to court, that team will deliver against that . . . I do not for a moment underestimate the complexity of this challenge or the potential emotional stress for relatives associated with revisiting these tragic events," he said.
"That is why families will sit at the very heart of our investigations, and that is why our primary objective will be to work with them to achieve some measure of resolution for them."
The team will be led by Cdr David Cox, who has retired from London's Metropolitan Police.
He said he understood that for many families the reopening of the cases involving the deaths of their loved ones could prove difficult. However, he hoped the discovery of any new evidence surrounding their loss could help everyone in Northern Ireland move on.
"That is why families will sit at the very heart of our investigations, and that is why our primary objective will be to work with them to achieve some measure of resolution for them," he said.
The new police investigation will employ the latest forensic and intelligence techniques and will be divided into four stages.
These were described as: collection and assessment, involving the recovery and examination of existing records and exhibits regarding an unsolved case; review - examination of cases to determine whether any further investigative or evidential opportunities exist; reinvestigation, further investigative work focusing on the issues identified by the review process; and judicial proceedings - in appropriate cases, where possible, legal proceedings will be initiated or alternatively there will be moves towards resolution.
Northern Secretary Peter Hain welcomed the beginning of the new inquiries into the murders. "All of them will be investigated and there will be closure on them," he promised.
However An Fhirinne, which represents the victims of those who died as a result of collusion between the security forces and loyalists, was critical.
Spokesman Robert McClenaghan said the new inquiries team would not resolve so many cases.
"Those accused of murder are being asked to reinvestigate themselves," he said.
"That is why the [ team] will fail to get to the truth about collusion. Only an independent, international inquiry will have the authority and confidence of victims' families to get to the truth about collusion and state murder," Mr McClenaghan said.