Sir Ronnie Flanagan, head of the Royal Ulster Constabulary, appointed Kent Chief Constable David Phillips, 54, to head the inquiry.

A Federal Bureau of Investigation agent from the United States is also flying in on Wednesday.

Tom Coulter reports: "An international investigation for Rosemary Nelson"

The response has failed to satisfy some nationalist politicians and human rights campaigners in the province.

Mrs Nelson, a prominent human rights lawyer in the province, died on Monday after a bomb destroyed her car in Lurgan, County Armagh.

Northern Ireland secretary Mo Mowlam has told an audience in Washington on the eve of St Patrick's Day that the peace process will not be derailed by the killing.

Sir Ronnie Flanagan: "I'm determined that this investigation will be transparent"

Sir Ronnie told BBC Radio Ulster he hoped the FBI's reputation would provide "an international dimension as well as added independence".

The RUC chief voiced concern over the improved capability of the bombers, the dissident loyalist Red Hand Defenders, saying the group had not previously demonstrated the "same ruthless capability".

Kent chief David Phillips will lead inquiry

The announcement follows Mrs Nelson's own allegations of collusion between elements of the security forces and loyalist terrorists in targeting nationalist figures.

Sir Ronnie has rejected the allegations and pledged to ensure they do not distract from his officers' "diligence and professionalism".

Meanwhile, detectives have revealed that Mrs Nelson's car had not been at her home all weekend, as was previously reported.

Law Society of Northern Ireland's Catherine Dickson: "Almost a direct attack on democracy itself"

They say she used it during a weekend family break in the Irish Republic, returning on Sunday night.

A police spokesman said: "They travelled in a silver BMW, registration BLZ 4584, which was then parked in the driveway of her home and was not moved until Mrs Nelson drove it at 12.40pm on Monday."

Detectives are asking anyone who saw any suspicious activity around the Ashford Grange and Lake Street areas of Lurgan to call the incident room on 01762 325144.

On Tuesday, flowers were laid at the spot where the car exploded.

Mrs Nelson's funeral takes place on Thursday and Irish Foreign Minister David Andrews will be among the mourners.

Threats and intelligence

Downing Street and the Northern Ireland Office have both denied Sinn Fein suggestions that the government refused the lawyer protection.

Mrs Nelson: Government says she never asked for protection

Ms Mowlam said Mrs Nelson would have been entitled to police protection, but said the issue was difficult "for people that don't want the RUC".

Sir Ronnie admitted personal security concerns had been raised - but not necessarily by Mrs Nelson herself.

He said a senior Metropolitan Police officer had investigated her claims of RUC officers making threats against her through clients in custody.

Audio and video recording of all interviews had been introduced as result and a file passed to the Director of Public Prosecutions, he added.

RUC 'incapable' - Adams

But in Washington Sinn Fein president Gerry Adams called Sir Ronnie's response "totally inadequate" and called for an independent international murder inquiry.

"The RUC was judged by the British Inspector of Constabularies to be incapable of impartially investigating the threats to Rosemary Nelson's life," he said.

"How can it now be capable of investigating her murder?"

And Irish-based human rights groups also in the US urged Ms Mowlam to remove RUC officers from the inquiry.

"It's essential the Kent chief constable has his own team," said Martin O'Brien, of the Committee on the Administration of Justice.

Meanwhile Jane Winter, of British/Irish Rights Watch feared some police officers regarded Mrs Nelson "as the enemy".