Ukrainian officials searched for answers on Thursday to the circumstances of a deadly plane crash over the border in Russia, asking for patience from citizens while it investigated Moscow’s claims that Ukraine had shot down a Russian military aircraft carrying Ukrainian prisoners of war.

President Volodymyr Zelensky of Ukraine called for his country’s intelligence agency to determine what had happened and for an international investigation into the crash, while also accusing Russia of engaging in wartime propaganda over the episode.

The plane went down in the Belgorod region near the border with Ukraine, killing all onboard, the Russian Ministry of Defense and the regional governor said on Wednesday. The ministry said that the plane had been carrying 65 Ukrainian prisoners of war who were to be released in a prisoner exchange. The Russian claims could not be independently verified.

The cause of the crash remains unknown, though Ukrainian officials have not denied shooting down the aircraft. They say they cannot confirm that Ukrainian soldiers on their way to a prisoner exchange were aboard the plane.

But the Ukrainian military intelligence agency on Wednesday hinted at the possibility of a tragic mistake, acknowledging that a prisoner swap had been planned and saying in a statement that Russia had not warned Ukraine’s military that prisoners of war were being transported to the border — as it had in previous exchanges.

Either way, the episode is a vexing challenge for a government in Kyiv trying to brace its citizens for a third year of war and contemplating a new mobilization that would likely be domestically unpopular. At the same time, Ukraine is on the defensive on the battlefield and striving daily to shore up critically needed Western support.