LVIV, Ukraine (AP) — Amid all the horrors that have unfolded in the war on Ukraine, the Russian airstrike on the theater being used as a bomb shelter in Mariupol on March 16 stands out as the single deadliest known attack against civilians to date.

An Associated Press investigation has found evidence that the attack was far deadlier than estimated, killing closer to 600 people. That’s almost double the current estimates.

The AP recreated what happened inside the theater on that day from the accounts of 23 survivors, rescuers, and people intimately familiar with the theater’s new life as a bomb shelter.

The AP also built a 3D model based on witness accounts, two sets of floor plans of the theater, photos and video taken inside before, during and after that day, and expert comment.