A security guard looks out from the entrance of the Saudi Arabia’s Consulate in Istanbul on Sunday. (AP Photo/Petros Giannakouris)

David Kaye is the United Nations Special Rapporteur on freedom of expression, and Agnes Callamard is the United Nations Special Rapporteur on summary executions.

Nearly two weeks ago, journalist Jamal Khashoggi stepped into the consulate of Saudi Arabia in Istanbul. He has not been seen since. The Turkish government has signaled that there is evidence connecting Saudi Arabia not only with his disappearance but also with his brutal murder. The Saudis have shown zero evidence or even a plausible argument of innocence in response. Saudi responsibility seems practically irrefutable.

Case closed, no? Unfortunately, we are far from that. Turkey’s investigation remains hidden in the murky shadows of intelligence and security services. Any criminal investigation that would naturally lead to accountability for murder would be difficult, subject to concerns of diplomatic pressure on Turkey. There is a big risk that this case could be further politicized. To avoid this, we think Turkey should not be leading the investigation. We say this not as a criticism of Turkey as much as a belief in the necessity to protect an effort to achieve truth.

Who then can “get to the bottom of it,” as Donald Trump put it, in a credible way? The Saudis cannot investigate themselves, obviously. Any investigation involving the United States or European governments would face the same type of politicization and diplomatic conflict as one led by Turkey. U.S. objectivity would be particularly in question given the Trump administration’s close relationship with the current Saudi leadership.

There are at least two alternatives, as we see it, and both involve Turkey’s consent. One is that Turkey invites an international team of investigators to evaluate the evidence and allow a transparent disclosure and evaluation of the findings of Turkish authorities. Our fear is that such a process would still face criticism for depending on Turkey’s control of the evidence and access to information.

The other, better possibility could be to establish an emergency independent international team to conduct its own investigation. The U.N. Security Council should authorize an immediate investigation that would report back to it within the next four to six weeks. If any country blocks a Security Council-sanctioned investigation, the U.N. Human Rights Council should authorize one, as it has done for Syria, Myanmar, Burundi and elsewhere.

What might that investigation look like? There have been similar investigations in the past, and there’s ample drafting expertise for getting this right. We believe the importance of this issue calls for a Security Council authorization. Just last year the Security Council appointed an investigative team to look at the crimes of the Islamic State. In 2004, it authorized a key commission about crimes in Darfur.

At a minimum, an international investigation should involve the appointment, by the secretary general, of a high-profile and well-respected former international prosecutor or judge. That person would lead a small team of international investigators with global experience to examine the existing evidence, provided by Turkey, and have the authority to seek additional evidence. The resolution authorizing the investigation should welcome Turkey’s consent to full cooperation and compel Saudi Arabia to provide access to the consulate and to relevant officials and other sources.

We are not calling for an international tribunal but rather for an independent investigation that could produce credible findings and provide the basis for clear punitive actions, including the possible expulsion of diplomatic personnel, removal from U.N. bodies (such as the Human Rights Council), travel bans, economic consequences, reparations and the possibility of trials in third states.

There is another reason for the international community to engage in this way right now. Khashoggi’s disappearance comes at a moment of extreme threat to journalists worldwide. The Committee to Protect Journalists has identified 44 cases of journalists killed this year. A few days ago Bulgarian TV reporter Viktoria Marinova was raped and killed.

An international investigation would at least start to change that narrative. To be sure, several countries in the Security Council are dangerous places for journalism and free expression. An inquiry investigation cannot be cover for their own bad behavior.

But Khashoggi’s disappearance must lead to accountability and consequences. Only an independent investigation can put this case on that path.

Read more:

The Post’s View: We need answers on Jamal Khashoggi’s disappearance

David Ignatius: Jamal Khashoggi chose to tell the truth. It’s part of the reason he’s beloved.

Read Jamal Khashoggi’s columns for The Washington Post

Manal al-Sharif: Jamal Khashoggi’s disappearance spreads fear worldwide, but we won’t be silenced