Four of the parties in Northern Ireland have called on the British government to hold a public inquiry into the murder of Belfast solicitor Pat Finucane in 1989.
It is one of the most controversial killings of the Troubles in Northern Ireland.
The 39-year-old was shot by loyalist paramilitaries in front of his family in 1989 amid claims of collusion between the killers and security forces.
In February last year, the UK Supreme Court ruled that investigations into the murder have not been effective and fell short of international human rights standards.
Northern Secretary Brandon Lewis has said he will make a decision about whether a public inquiry will be held before the end of this month.
Sinn Féin, the SDLP, Alliance and Green Party have sent a joint letter to Mr Lewis urging him to "act in the public interest" by holding a public inquiry.
It is signed by Stormont Deputy First Minister and Sinn Féin leader in Northern Ireland Michelle O'Neill, SDLP leader Colum Eastwood, Stephen Farry MP of the Alliance Party, and Claire Bailey, leader of the Green Party in the north.
It states: "The former British Prime Minister David Cameron acknowledged 'shocking levels of collusion' in the killing of Pat Finucane, a much-loved brother, husband and father at his family home in North Belfast on 12 February 1989.
"Pat was also a distinguished human rights lawyer.
"In February 2019, the Supreme Court ruled that all previous investigations were incapable of establishing the full facts into the killing.
"Thirty years on, it remains a matter of the utmost public interest that questions remain unanswered about who ordered the killing, about who knew what, and when.
"Only a public inquiry can now assist the Finucane Family to get the truth about the scope and extent of state collusion into Pat's killing.
"How the British government responds to the Supreme Court is also now a matter of public interest. Holding a public inquiry provides the British government with an opportunity to restore wider confidence in the rule of law and the administration of justice.
"We are therefore calling on you, and the British government, to act in the public interest and hold a Public Inquiry into the 1989 killing of Pat Finucane."