The move came after a wave of criticism and allegations against the RUC from nationalist and human rights sources following the death of Mrs Nelson from injuries inflicted by an undercar boobytrap bomb on Monday.
With accusations of collusion thick in the air, the force clearly believed it had to move swiftly to counter criticisms that it would be investigating itself.
The Chief Constable, Sir Ronnie Flanagan, announced he had asked the Chief Constable of Kent, David Phillips, to head the investigation, and that the FBI would also be involved.
He added: "I think it is important, when there are grave concerns about Mrs Nelson on the international front, that their involvement will also give this investigation an international dimension as well as added independence.
"I recognise this is an unprecedented case in the nature of the sort of allegations that have been made."
The Northern Ireland Secretary, Mo Mowlam, in the United States for meetings and St Patrick's Day celebrations, led a minute's silence for Mrs Nelson in Washington yesterday.She described Mrs Nelson as "a brave woman who stood up for what she believed in and carried those beliefs into her work".
Although the murder was claimed by the Red Hand Defenders, a small grouping responsible for two other killings in the past six months, the possibility that it had help from a larger loyalist organisation is being investigated. Undercar boobytraps were a regular feature of the IRA's campaign of violence until the early Nineties, but they have remained outside the capabilities of almost all loyalist organisations. The exception to this has been the Ulster Defence Association, which has mounted four such attacks in just over two years, one of which resulted in a death. The possibility of a large-scale return to UDA violence is of major concern to the authorities.
In Lurgan, Co Armagh, parish priest Father Kieran McParlan said Mrs Hamilton's husband and three children were shocked and distressed by the killing. "They are numb, feeling very helpless and in need of support and care."
In the city of Londonderry, solicitors staged a march to an RUC station. One of them, Paddy McDermott, said they wished to express disquiet at the murder and solidarity with the Nelson family.
"We wish to make it clear that we want to see a full and proper independent investigation into Rosemary's death and into the threats which were made against her life."
Param Cumaraswamy, the United Nations special investigator to whom Mrs Nelson had complained of police intimidation, said he had feared for her life. Describing her as a very courageous human rights lawyer, he added: "Though I feared that Rosemary's life was at stake, I really didn't expect this to happen to her and this is really a shock."
On 30 March, Mrs Nelson had been due to meet the Independent Police Complaints Commission to hear the results of an investigation into her complaints that she had been the subject of death threats and assaults from officers of the RUC. The Commission had supervised an inquiry by the Metropolitan Police into her allegations and the police report is now with the Director of Public Prosecutions in Northern Ireland.
The UN report which prompted the investigation was the latest in a long line of inquiries into human rights issues in Northern Ireland which describe a pattern in which RUC detectives questioning paramilitary suspects are said to threaten both the suspects and their lawyers.Reuse content