PRESIDENT RODRIGO DUTERTE of the Philippines is not introspective about the violence he has unleashed against supposed drug users and dealers. The extrajudicial killings have resulted in at least 5,000 deaths, perhaps thousands more, with no due process. Speaking in Tokyo last month, Mr. Duterte was unapologetic. “I don’t care how many of you have died,” he said, “just don’t f--- with me on drugs.”

This brazen refusal to be held to account for a slaughter is one more reason to take seriously the need for an impartial and outside investigation, which Mr. Duterte has repeatedly rebuffed. In recent days, 11 human rights special rapporteurs at the United Nations have called on the body’s Human Rights Council to start such an investigation. This is a potential case of crimes against humanity, and only a thorough, independent probe can begin to establish the facts.

“We have recorded a staggering number of unlawful deaths and police killings in the context of the so-called war on drugs, as well as killings of human rights defenders,” the special rapporteurs said in a statement. “Very few independent and effective investigations have taken place, independent media and journalists are threatened, the law has been weaponised to undermine press freedom, and the independence of the judiciary is undermined. We are extremely concerned over the high number of killings which are being carried out across the country in an apparent climate of official, institutional impunity.”

The group said it has raised concerns 33 times with the Philippines during the past three years about “a range of gross human rights violations,” including extrajudicial, summary or arbitrary executions. The drug war Mr. Duterte declared after taking office in 2016 has involved police and motorcycle-riding vigilantes shooting to kill people they think might be drug users or dealers, leaving families to grieve without any accountability for the death squads. More recently, there have been indications that Mr. Duterte, who effectively received a vote of confidence in the recent midterm elections, is expanding the use of drug-war tactics in other areas, such as against political activists. He has also sought repeatedly to squelch free speech.

Mr. Duterte pulled the Philippines out of the International Criminal Court in March after it launched a preliminary inquiry into the extrajudicial killings of his drug war. He quickly rejected the latest call for a U.N. investigation, with his government saying the Philippines is a democracy with rule of law, rejecting “unpardonable intrusions” and saying the criticism was a “biased and absolutely false recital of facts.” Considering that the 47-member Human Rights Council has a number of members with records of gross violations, such as China, Cuba, Egypt and Saudi Arabia, the prospects for launching a serious Philippines investigation are challenging. The Philippines is also on the council. But the proposal is a good one. A commission of inquiry such as the one that probed North Korea a few years ago can have lasting impact and ensure that crimes against humanity are not hushed up.

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