A murder inquiry into the Bloody Sunday killings in Co Derry is to begin in the New Year.
Senior commanders from the PSNI briefed relatives of the 14 people who died after British paratroopers opened fire on civil rights demonstrators in the city in 1972.
Earlier this year, police signalled an intent to investigate the incident after they and prosecutors reviewed the findings of the Saville public inquiry into the controversial shootings.
Until today it had been unclear when such a probe would start.
After the 12-year inquiry, Mr Saville found that the killings were unjustified and none of the dead posed a threat when they were shot.
That contradicted the long-standing official version of events, outlined in the contentious 1972 Widgery report, which had exonerated soldiers of any blame.
The Saville Inquiry prompted Prime Minister David Cameron to apologise to the relatives.
Thirteen people died on the day, with a badly injured man dying a number of months later.
PSNI Deputy Chief Constable Judith Gillespie and Assistant Chief Constable Drew Harris outlined details of the murder investigation to the Bloody Sunday families in the city this morning.
Police have said the investigation will be "lengthy and complex". It is expected to last at least four years.
Statements given by witnesses to the Saville Inquiry cannot be used as evidence in criminal proceedings.
Police have therefore urged those people to come forward again to make official statements.
"Senior police met a delegation in Derry today representing some of the victims killed on Bloody Sunday," said a PSNI spokeswoman.
"Following consultation earlier this year with the Public Prosecution Service, officers provided an outline of the processes involved and the challenges posed by a criminal investigation into the events of 30 January 1972.
"The delegation was informed about the appointment from Serious Crime Branch of a senior investigating officer and the allocation of resources to create an investigation team which will begin work in the New Year.
"For the investigation to be as comprehensive and effective as possible, police will be asking for public support in the form of witnesses who gave evidence to the Saville Inquiry now making statements to detectives.
"This is because police are precluded from using Saville testimony in a criminal investigation. Details on how this process will be facilitated will be made available in the near future.
"Police have also undertaken to provide updates to surviving victims and all the families who lost relatives on Bloody Sunday through the course of the investigation which will be lengthy and complex."
Elsewhere, Northern Police Ombudsman Dr Michael Maguire has accepted a court direction that his office should set aside its public report into aspects of the terrorist attack on Loughinisland in 1994, which was issued in June 2011.
In a statement, the ombudsman said: “In the light of the Judicial Review proceedings instigated on behalf of Brigid Green, Dr Maguire commissioned a review of the content of the report.
“Having considered the findings of that review, he accepts that there may be deficiencies in specific areas of the report and is therefore able to accept the court direction setting it aside.”