An investigation into the Omagh bomb is to be debated in the Northern Ireland Assembly on Tuesday.
The Alliance Party motion calls for a legally binding cross-border investigation, not a full inquiry as relatives of some of the victims want.
The 1998 Real IRA car bomb killed 29 people.
Party leader David Ford said "the people of Omagh have a right to know the full facts" about the attack.
"This bombing resulted in the biggest loss of life throughout the whole of the Troubles and any right-thinking person should support this call for total disclosure of all information about the incident," he said.
"Clarity and a full account of events are the least that those affected by the bomb deserve."
The Alliance motion calls for a process "designed to secure full disclosure from the intelligence services and security forces in the United Kingdom and the Republic of Ireland, to establish what relevant information they had before and immediately after the attack".
Michael Gallagher, chairman of the Omagh support and self-help group, said a public inquiry into the attack was important to everyone "on the island of Ireland and beyond".
"We should learn what was done well and what was not done well, and those lessons should be passed on to others so that we are better prepared for any future terrorist attack or disaster," he said.
"The inquiry should never take away from those who were responsible for planning and preparing this evil act."
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