Ms Hillary Rodham Clinton said in Belfast yesterday it was imperative those behind the killing of solicitor Rosemary Nelson were brought to justice.
In an address to the Vital Voices women's conference, Ms Clinton said she welcomed the continued moves to peace since her last visit eight months ago, but understood there had been setbacks.
She said the murder of the Lurgan-based solicitor in March represented not only an attack on an individual but was also an attack on the rule of law. It was "an effort to derail the common efforts that people of both traditions and throughout the community have been making to bring about peace and inclusion in Northern Ireland."
She said the murder must "stand as a final rebuke to all who would try to derail the peace process."
Ms Clinton received a standing ovation from the 400 delegates attending the Vital Voices conference, an organisation she helped launch during her last trip to the North. She heard an update on projects started in recent months, covering areas such as childcare, job-share and flexitime schemes which aim to get more women into the workforce.
A further 800 women from various voluntary and community groups gathered in Craigavon, Co Armagh, Derry and Cavan, and were connected to the Belfast conference by satellite.
Also addressing the conference was the North's First Minister, Mr David Trimble, who said he was frustrated by the slow political progress in the North when it was contrasted to devolution in Scotland and Wales.
The North's Deputy First Minister, Mr Seamus Mallon, called on the people of Northern Ireland to "mobilise" for peace. "The task that faces us all is to transform the quiet whisper of conscience into a powerful and vital voice."