The Police Service of Northern Ireland is to launch a murder investigation into the deaths of 13 people shot dead by British soldiers in Derry on Bloody Sunday in 1972.
However, the police warned the murder inquiry could take up to four years.
The investigation has not started but would follow the Saville inquiry's report, which said civil rights demonstrators shot dead by soldiers in Derry at the height of the Troubles were innocent.
The PSNI and Public Prosecution Service have reviewed the Saville findings, which was published in June 2010, after a 12-year inquiry.
It is expected that the investigation would require a team of 30 officers and extra specialist resources.
PSNI chief constable Matt Baggott said: "I do not think anywhere else in the world is facing the challenges of organised crime, paramilitary activity ... alongside having to deal with 30 years of misery in such a way."
Thirteen people were shot dead when soldiers opened fire on marchers during a civil rights march in Derry on January 30th, 1972. Another man John Johnston, died five months later from gunshot wounds.
A report by Lord Saville blamed the British army for one of the most controversial days in Northern Ireland's history.
Key findings included:
No warning had been given to any civilians before the soldiers opened fire
None of the soldiers fired in response to attacks by petrol bombers or stone throwers
Some of those killed or injured were clearly fleeing or going to help those injured or dying
None of the casualties was posing a threat or doing anything that would justify their shooting
Many of the soldiers lied about their actions
Northern Ireland's Deputy First Minister Martin McGuinness, was present at the time of the violence and "probably armed with a sub-machine gun" but did not engage in "any activity that provided any of the soldiers with any justification for opening fire".
Mr Baggott said matters contained in the report should be investigated but asked what the consequences were for keeping people safe now if detectives were diverted from today's crimes.
"I cannot ask the people doing this to take on a whole raft of other tasks which may be serious by themselves," he told the Policing Board in Belfast.
Assistant chief constable Drew Harris said it had already been a long and resource-intensive investigation considering preliminaries with the prosecution service and added the matter would need to be brought to the Policing Board which oversees policing in Northern Ireland to assess priorities.
He said police needed to strike a balance between protecting life in the present day and the need to investigate historic crimes, so-called legacy issues from Northern Ireland's long conflict.
Controversial killings from the conflict have resulted in 46 inquests being heard, which require police input, an investigation into the death of solicitor Pat Finucane in 1989 and co-operation with the independent Historical Enquiries Team which is looking at all unresolved cases.
Police are presenting a report to the Policing Board on legacy issues in October.
Mr Baggott said the Public Prosecution Service (PPS), which takes court cases, had provided a view on suspected criminal offences during Bloody Sunday and the areas where investigation was warranted. Material from the Saville report is excluded from criminal proceedings so any investigation effectively starts from scratch.
Mr Harris said: "That will be a large investigation obviously and setting aside the resources to properly start that and take that forward is a corporate issue which is under investigation at this time."
He said the probe had not yet begun and police would need to comply with competing parts of the Human Rights Act.
Mr Harris added: "There is not the expertise free and available to undertake an investigation of this size and that is why we are faced with dilemmas around prioritisation.
"The special resources required for this scale of investigation are just not available at this moment to commence an investigation of this scale and length of time. It is an undertaking which will take perhaps three to four years."
PA