The Associated Press says it has conducted an investigation that shows about 600 civilians died when Russia attacked a theater in the port city of Mariupol that was being used as a bomb shelter.

The exact number of deaths as a result of the March 16 air strike on Donetsk Academic Regional Drama Theater, which was clearly marked outside as being a place where children were located, has been unknown as heavy fighting has kept authorities from properly sifting through the debris.

While Ukrainian officials have said at least 300 people died in the attack, AP said that, in a recreation of the events based on the accounts of 23 survivors, rescuers, and people intimately familiar with its new life as a bomb shelter, the figure is at least twice as high.

The news agency said it also drew on two sets of floor plans of the theater, photos and video taken inside before, during, and after that day, and feedback from experts who reviewed the methodology. It also used 3D modeling to recreate the situation.

While Russia has denied targeting civilians since it launched its unprovoked attack on Ukraine on February 24, mounting evidence tells a different story. The air strike on the theater in Mariupol stands out as the single deadliest known attack against civilians to date.

“All the people are still under the rubble, because the rubble is still there — no one dug them up,” AP quoted Oksana Syomina, who survived the deadly attack, as saying.

“This is one big mass grave.”