KYIV, Ukraine (AP) — Russia and Ukraine accused each other Friday of shelling a prison in a separatist region of eastern Ukraine, an attack that reportedly killed dozens of Ukrainian prisoners of war captured after the fall of Mariupol, the city where troops famously held out against a monthslong Russian siege.

Both sides said the assault was premeditated with the aim of covering up atrocities.

Russia claimed that Ukraine’s military used U.S.-supplied rocket launchers to strike the prison in Olenivka, a settlement controlled by the Moscow-backed Donetsk People’s Republic. Separatist authorities and Russian officials said the attack killed 53 Ukrainian POWs and wounded another 75.

Moscow opened a probe into the attack, sending a team to the site from Russia’s Investigative Committee, the country’s main criminal investigation agency. The state RIA Novosti agency reported that fragments of U.S.-supplied precision High Mobility Artillery Rocket System rockets were found at the site.

The Ukrainian military denied making any rocket or artillery strikes in Olenivka, and it accused the Russians of shelling the prison to cover up the alleged torture and execution of Ukrainians there. An adviser to Ukrainian President Volodymyr Zelenskyy described the shelling as “a deliberate, cynical, calculated mass murder of Ukrainian prisoners.”

Neither claim could be independently verified.

Video shot by The Associated Press showed charred, twisted bed frames in the wrecked barracks, as well as burned bodies and metal sheets hanging from the destroyed roof. The footage also included bodies lined up on the ground next to a barbed-wire fence and an array of what was claimed to be metal rocket fragments on a wooden bench.

Denis Pushilin, the leader of the internationally unrecognized Donetsk republic, said the prison held 193 inmates. He did not specify how many were Ukrainian POWs.