Evidence suggests a bombing at a theater in Mariupol in March was twice as deadly as previously believed, a new investigation found.

The airstrike allegedly carried out by Russian forces on the building, which served as the main bomb shelter for the Ukrainian city, killed close to 600 people despite early government estimates suggesting 300 people died in the attack, according to the Associated Press.

"All the people are still under the rubble because the rubble is still there — no one dug them up," Oksana Syomina said, recalling the horrors of seeing bodies on the ground after the attack. "This is one big mass grave."

Around 1,000 civilians were believed to be in the building, while survivors reported they only saw about 200 people escape from the building. At least 100 people were reportedly at a field kitchen outside the shelter at the time. None of them survived, according to witnesses.

RUSSIAN FORCES BOMBED MARIUPOL THEATER WITH HUNDRED OF CIVILIANS, CITY COUNCIL SAYS

The estimated death toll was concluded through interviews with more than 23 people who either survived or had knowledge of the attack, recreated maps showing where civilians would have been in the building at the time, and more data.

The bombing has spurred a war crimes investigation by the Ukrainian government and has stood out as one of the gravest casualties of the war. Satellite footage before the attack appears to show the word "CHILDREN" written in large letters outside the building, seemingly notifying opposing forces that children were inside.

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Mariupol has been a target of massive shelling since the start of the invasion and one of the hardest-hit cities as Russian forces attacked schools, hospitals, and other civilian areas.