Ukrainian authorities found a mass burial site near a recaptured northeastern city previously occupied by Russian forces, President Volodymyr Zelenskyy announced Thursday night.

The grave was discovered close to Izium in the Kharkiv region.

“The necessary procedures have already begun there. More information — clear, verifiable information — should be available tomorrow,” Zelenskyy said in his nightly televised address.

Associated Press journalists saw the site Thursday in a forest outside Izium. Amid the trees were hundreds of graves with simple wooden crosses, most of them marked only with numbers. A larger grave bore a marker saying it contained the bodies of 17 Ukrainian soldiers.

Investigators with metal detectors were scanning the site for any hidden explosives.

Oleg Kotenko, an official with the Ukrainian ministry tasked with reintegrating occupied territories, said videos that Russian soldiers posted on social media indicated there were likely more than 17 bodies in the grave.

“We haven’t counted them yet, but I think there are more than 25 or even 30,” he said.

Izium resident Sergei Gorodko said that among the hundreds buried in individual graves were dozens of adults and children killed in a Russian airstrike on an apartment building.