Ukraine's SBU security service on Thursday opened a criminal investigation into the downing of a Russian military plane that Moscow said killed 65 Ukrainian prisoners of war.Kyiv has not said whether captured Ukrainian soldiers were killed or whether it was involved in Wednesday's shooting down of a military transport plane over Russia's western Belgorod region, close to the border with Ukraine."The security service of Ukraine has opened a criminal investigation into the downing of the IL-76 Russian Air Force plane in the Belgorod region," the SBU press service said."The SBU is currently taking a range of measures to clarify all the circumstances of the downing," it said.Ukrainian President Volodymyr Zelensky on Wednesday called for an international investigation into the incident.Kyiv has confirmed a prisoner exchange was due to take place on Wednesday at the border between the two countries but said Moscow did not inform them that any prisoners of war would be transported by plane.It has also not confirmed or denied Russia's claim that Ukrainian forces shot down the plane, killing dozens of Ukrainian soldiers on board.The SBU said its criminal probe would look into possible "violations of the laws and customs of war."Zelensky warned earlier on Thursday that Ukraine's ability to ascertain facts surrounding the crash would be hampered given that it happened on Russian territory.