U.S. intelligence officials have found that Russia is trying to fabricate evidence to make it appear that Ukrainian forces were responsible for the July 29 attack on Olenivka prison that left 53 dead and dozens wounded, a U.S. official familiar with the findings told The Associated Press of intelligence services. Press on Wednesday. Russia claimed that Ukraine’s military used US-supplied rocket launchers to hit the prison in Olenivka, a settlement controlled by the Moscow-backed Donetsk People’s Republic. 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 main KGB successor agency, and the Wagner mercenary group to mine the barracks before “using an incendiary substance, which led to the rapid spread of the fire in the room.” The official, who was not authorized to comment publicly and spoke on condition of anonymity, said classified information — which was recently downgraded — indicated that Russian officials may even field munitions from medium-range artillery missile systems, or HIMARS. evidence that US-supplied systems to Ukraine were used in the attack. Russia is expected to take action as it expects independent researchers and journalists to eventually gain access to Olenivka, the official added. Ukraine has effectively deployed HIMARS launchers, which launch medium-range rockets and can be moved quickly before Russia can target them with a response, and is seeking more launchers from the United States. Earlier on Wednesday, United Nations Secretary-General Antonio Guterres said he was appointing a fact-finding mission in response to requests from Russia and Ukraine to investigate the prison killings. Guterres told reporters that he does not have the authority to conduct criminal investigations, but he does have the authority to conduct fact-finding missions. He added that the terms of reference for a mission to Ukraine are currently being prepared and will be sent to the governments of Ukraine and Russia for approval. Ukrainian prisoners in the Donetsk prison included troops captured during the fall of Mariupol. They spent months hiding with civilians in the giant Azovstal steelworks in the 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 abandoned their fight and surrendered on the orders of 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. — Lederer reported from the United Nations. Associated Press writer Zeke Miller contributed to this report.