Some of the victims had been killed by shelling and air strikes, Serhiy Bolvinov, the chief police investigator for Kharkiv region, told Sky News.
Forensic investigations would be carried out on every body, he said.
"I can say it is one of the largest burial sites in a big town in liberated (areas) ... 440 bodies were buried in one place," Bolvinov said.
"Some died because of artillery fire ... some died because of air strikes," he said.
Thousands of Russian troops fled Izium at the weekend as Ukraine's forces recaptured the eastern town.
President Volodymyr Zelenskiy put the blame on Russia and likened the discovery to what happened in Bucha, on the outskirts of the capital Kyiv in the early stages of the late February invasion by Russian forces.
Ukraine and its Western allies have accused the Russian forces of perpetrating war crimes there.
"Russia is leaving death behind it everywhere and must be held responsible," Zelenskiy said in a video address.
Russia has repeatedly denied it targets civilians or has committed war crimes during what it calls its "special operation" in Ukraine, which began when troops crossed the border on February 24.
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