Antonio Guterres, the UN secretary-general, told reporters he did not have the authority to conduct criminal investigations but could conduct fact-finding missions, and terms of reference were being prepared for the Ukrainian and Russian governments to approve. The mission was created at the request of Russia and Ukraine. Russia claimed that Ukraine’s military used US-supplied rocket launchers to hit the prison in Olenivka, a settlement controlled by Russian-backed separatists. The attack killed 53 Ukrainian prisoners and wounded 75 others, separatist authorities and Russian officials said. The Ukrainian military has denied launching any rocket or artillery attack on Olenivka. The intelligence arm of Ukraine’s defense ministry claimed in a statement on Wednesday that it has evidence that local Kremlin-backed separatists colluded with Russia’s FSB, the KGB’s main successor agency, and the Wagner mercenary group to mine the barracks before “using an incendiary substance. which led to the rapid spread of fire in the room.” The Ukrainian military on Tuesday also claimed the barracks had been blown up from the inside, citing the nature of the damage it said was inconsistent with Russian claims that Ukraine had bombed the building. These claims could not be immediately verified. The US, meanwhile, believes Russia is preparing to fabricate evidence pointing the finger at Ukraine. A U.S. intelligence official told The Associated Press that recently declassified material showed Russian officials may plant munitions from high-mobility medium-range artillery missile systems, or Himars, as evidence that systems provided by the U.S. to Ukraine were used in the attack. Guterres said he took “very seriously” Russia’s and Ukraine’s requests for a UN investigation into last Friday’s attack and expressed hope that both countries would agree to the terms of reference. At the same time, he said, the UN was looking for “competent, independent people” to join the mission. The UN chief also expressed hope that warring countries would facilitate the mission’s access and provide the data needed “to clarify the truth about what happened.” Ukrainian POWs in Donetsk prison included troops captured during the fall of Mariupol. They spent months hiding with civilians in the giant Azovstal steelworks in the strategic southern port city. Their resistance during a relentless Russian bombardment became a symbol of Ukrainian resistance against Russian aggression. More than 2,400 soldiers from the Azov regiment of the Ukrainian national guard and other military units surrendered following orders from the Ukrainian military in May. Dozens of Ukrainian soldiers were taken to prisons in Russian-controlled areas. Some have returned to Ukraine as part of prisoner exchanges with Russia, but other families have no idea if their loved ones are still alive or if they will ever return home. Ukraine’s defense ministry claimed on Wednesday that Ukrainian prisoners in the prison had been subjected to “intimidation, physical humiliation and psychological degradation” in an attempt to force them to star in pro-Russian propaganda videos. “Ukrainian prisoners showed extraordinary courage and indomitable will,” the ministry said, arguing that Moscow and the separatists did not intend to include the prisoners in exchange and chose to “deliberately destroy” them in order to hide signs of torture that could be used as evidence in international criminal proceedings. He did not immediately disclose how he arrived at that rating.