U.N. Secretary General António Guterres has the legal power under the U.N. Charter to create an investigatory panel. (Khalil Senosi/AP)

Steven Ratner is a professor of law at the University of Michigan and was a member of the U.N. secretary general’s panel of experts on accountability in Sri Lanka.

More than three months have passed since the assassination of the journalist and Washington Post columnist Jamal Khashoggi at the Saudi Consulate in Istanbul. Although U.S. and Turkish intelligence agencies have concluded that the killing was ordered by senior Saudi officials, the kingdom’s leaders still deny involvement in the operation. Khashoggi’s body has yet to be found. Instead of truth and accountability, Saudi Arabia is trying 11 unnamed lower-level officials, who have no incentive to tell what happened and some of whom may be executed to keep them from ever doing so. It’s not even a show trial because it’s secret.

Yet there is still one possibility for finding the facts and holding the murderers accountable: a United Nations-led investigation convened by Secretary General António Guterres. The United Nations has demonstrated repeatedly that it can assemble teams of independent law enforcement, forensic and legal experts to investigate atrocities and produce a report that has international credibility. Syria, North Korea, Sudan, South Sudan and the Central African Republic have been on the receiving end of these inquiries. Past investigations have addressed assassinations, notably of Pakistan’s Benazir Bhutto and Lebanon’s Rafiq al-Hariri. When the results are published, governments and individuals behind these crimes have a hard time denying to the world what they have done. States can face diplomatic and economic sanctions, and individuals risk travel bans and asset freezes.

Most of the United Nations' inquiries were approved by a political body such as the Security Council or Human Rights Council. But the secretary general can set up an investigation into Khashoggi’s murder on his own. In 2009, after the end of Sri Lanka’s civil war, its government insisted that it had not killed any civilians and refused to investigate atrocities. Sri Lanka’s allies prevented the United Nations' political organs from convening an inquiry. So Secretary General Ban Ki-moon courageously created a panel of experts, on which I served, to gather allegations of crimes and recommend steps for the government. When we published our report in 2011, including credible claims of government attacks on thousands of Tamil civilians, the regime’s wall of denial came crumbling down. Worldwide condemnation followed; domestic opposition eventually led to the regime’s defeat at the polls. Those involved avoid travel because of the possibility of trials abroad.

Saudi Arabia has enough friends, including a powerful one in Washington, so the Security Council and Human Rights Council are unlikely to act in the case of Khashoggi. A request from Turkey to the United Nations to assist its investigation would offer political cover to the world body, but for reasons still a bit unfathomable, Turkey’s president hasn’t asked for help. No matter. Guterres and his lawyers know that he has the legal power under the U.N. Charter to create an investigatory panel even without a request. He should do so now.

Indeed, Khashoggi’s murder justifies a U.N. investigation even if we set aside the flagrant attack on journalism. Saudi Arabia defied two core rules of the international system: the ban on extraterritorial killings in peacetime and the requirement that states use diplomatic missions for official purposes, in exchange for which those missions get diplomatic immunity. Both rules are mostly honored (though with some notable violations by Iran, Russia and Israel over the years). They assure states that foreign agents will not commit crimes that might hurt their own citizens, and they allow embassies and consulates to keep open channels of diplomacy even when the sending countries have tense relations. Using a consulate as a killing chamber is an unprecedented attack on these rules. Guterres can easily claim that this is a unique, compelling case for him to act alone. He will likely find broad support for the inquiry from the United Nations' members. Delegations to the United Nations in New York should remind Guterres of this fact.

How could a U.N. investigation succeed without Saudi cooperation? Skilled investigators can locate witnesses and evidence; they can get information from states through assurances not to share intelligence sources and methods. It might also produce Khashoggi’s remains or an account of their destruction. Those it identifies as behind the killing may not face prosecution or political defeat at home, but they certainly can become international pariahs. Any Saudi refusal to cooperate with the inquiry will only heighten suspicions of state complicity.

Guterres has a chance to demonstrate his moral leadership in the face of the murder of an independent journalist and the importance of respect for key rules of the international order. He has the power to do so. Now is the time for him to act, so we can finally find out the facts.

Read more:

The Post’s View: Saudi Arabia’s trial for Khashoggi’s murder is a travesty. Congress must insist on justice.

David Ignatius: The Saudi engine of repression continues to run at full speed

Kenneth Roth: The Saudi crown prince should fear the long reach of justice

Jamal Khashoggi: What the Arab world needs most is free expression