LVIV, Ukraine (AP) — She stood in just her bathrobe in the freezing basement of the Mariupol theater, coated in white plaster dust shaken loose by the explosion. Her husband tugged at her to leave and begged her to cover her eyes.

But she couldn't help it — Oksana Syomina looked. And to this day, she wishes she hadn't. Bodies were strewn everywhere, including those of children. By the main exit, a little girl lay still on the floor.

Syomina had to step on the dead to escape the building that had served as the Ukrainian city's main bomb shelter for more than a week. The wounded screamed, as did those trying to find loved ones. Syomina, her husband and about 30 others ran blindly toward the sea and up the shore for almost five miles (eight kilometers) without stopping, the theater in ruins behind them.

"All the people are still under the rubble, because the rubble is still there — no one dug them up," Syomina said, weeping at the memory. "This is one big mass grave."

Amid all the horrors that have unfolded in the war on Ukraine, the Russian bombing of the Donetsk Academic Regional Drama Theater 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 in fact far deadlier than estimated, killing closer to 600 people inside and outside the building. That's almost double the death toll cited so far, and many survivors put the number even higher.

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

With communications severed, people coming and going constantly, and memories blurred by trauma, an exact toll is impossible to determine. The government estimated early on that about 300 people died and has since opened a war crimes investigation, according to a document obtained by the AP.

AP journalists arrived at a much higher number through the reconstruction of a 3D model of the building's floorplan reviewed repeatedly by direct witnesses, most from within the theater, who described in detail where people were sheltering.

All the witnesses said at least 100 people were at a field kitchen just outside, and none survived. They also said the rooms and hallways inside the building were packed, with about one person for every 3 square meters of free space.

Many survivors estimated around 1,000 people were inside at the time of the airstrike, but the most anyone saw escape, including rescuers, was around 200. The survivors primarily left through the main exit or one side entrance; the other side and the back were crushed.

The AP investigation also refutes Russian claims that the theater was demolished by Ukrainian forces or served as a Ukrainian military base. None of the witnesses saw Ukrainian soldiers operating inside the building. And not one person doubted that the theater was destroyed in a Russian air attack aimed with precision at a civilian target everyone knew was the city's largest bomb shelter, with children in it.

James Gow, a professor of international security at King's College London, said documenting what happened at the theater is critical to establishing a pattern of crimes against humanity in Ukraine.

"This strong witness testimony will be important in establishing that (Russian illegal) conduct was widespread or systematic," said Gow, who also served as an expert witness at the U.N. International Criminal Tribunal for the former Yugoslavia.

Mariupol has taken on outsize importance as a symbol of the devastation inflicted by Russian forces and of the resistance from Ukraine. The city's fate is now hanging in the balance, and officials say around 20,000 civilians died during the Russian siege. With Mariupol cut off from access, many fear the bombing of the theater presages more war crimes that have yet to be discovered.

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The elegant theater had stood in a square in the heart of Mariupol for more than 60 years, a stone building with white pillars, a classical frieze, and a distinctive red roof. It was once called the Russian Dramatic Theater, but local authorities removed the word "Russian" from the name in 2015. Last July, they ordered all performances to be conducted in Ukrainian.

The Russian siege of Mariupol started in the first days of March. The actors, designers and administrators who ran the theater took refuge there a few days later, on March 5. About 60 people spread out in a building with an audience capacity of 600, according to Elena Bila, who was a stage manager there for 19 years.

The city soon ordered the entire building opened as a bomb shelter, given its size, its unusually sturdy walls and its large basement. On the first day, about 600 people showed up, Bila said.

Every day, more and more people came, and they settled in the corridors. A group of 16 men formed a security committee, taking shifts to guard the front doors.

"When people came in, they thought they were safe," Bila said. "In fact, they weren't safe."

About a week before the bombing, the theater's set designer used white paint to inscribe the word "CHILDREN" in Cyrillic letters on the pavement outside, in the hope of staving off an attack from above. The signs, painted in both the front and back entrances, were large enough to be read even from satellites.

On March 9, a Russian airstrike hit a maternity hospital just a few blocks away, and two or three pregnant women moved to the theater for safety, according to two theater employees. The women, along with families with small children, were given the most comfortable dressing rooms on the second floor, along a corridor behind the stage. It would turn out to be their doom.

By March 15, around 1,200 people crammed into the building, sleeping in offices, corridors, balconies, the basement. They lined the curved hallways and the warren of backstage offices and dressing rooms. They sat in the auditorium on once-plush seats whose stuffing was used as kindling for cooking fires.

But they avoided sleeping on the stage, which sat beneath a domed ceiling and felt uncomfortably like the bullseye it turned out to be. Only pets — cats and dogs — were kept there, directly under the dome. The cavernous basement prop room beneath it was empty.

By this time, the city no longer had electricity, food and water. The theater became a place where anyone could get food and water supplied by the Red Cross or news about possible evacuations. A water tank stood out front, and the field kitchen operated to one side.

People also flocked to the theater as the most likely starting point for any evacuations, to get near the front of the line. New arrivals registered at the entrance, where the cloakroom used to be. Just past the registration was what served as a warm welcome: A stand with hot tea.

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Among those who showed up in the hope of evacuating on the morning of March 16 were the Kutnyakov family and their neighbors.

Any hesitation they might have had about abandoning their home evaporated when the building next door caught fire.

The six of them ran past a Russian tank, past a hospital already destroyed by shelling, then inadvertently toward another Russian tank, whose turret turned in their direction and opened fire. They hid briefly in the ruins of the children's clinic at the hospital. Then they ran down a side street for the final half-mile (kilometer) to the theater.

"We were immediately offered and poured tea," said Galina Kutnyakova, the 56-year-old matriarch. "You have to imagine, we had hardly eaten or drunk for six days. Everyone was so happy because of the hot tea."

Lunch was at noon, they were told, and in the meantime, they could find space.

The basement was full already. So were the first and second floors. They saw a spot on the third floor, near enormous windows that everyone knew would surely shatter into knives of flying glass if the building was hit.

It was the only place available, so they took it. They swept it up with a broom and laid out the sheets they'd grabbed from home. It was just before 10 a.m.

Maria Kutnyakova, Galina's 30-year-old daughter, walked through the entire building in search of free space, noting the full rooms. She left her mother to handle the registration and went out by herself to find her uncle, who lived nearby. They hadn't seen him in nine days.

That's when she heard warplanes flying in from the sea and heading to the Azovstal steel plant. She walked a little further, and heard a single plane, much closer.

Then came the explosion. As she hugged the edge of the nearest building, she thought to herself, "So it exploded. Let it explode. I've heard a million bombs like that, and the bottom line is it didn't hit me."

But she saw smoke rising from the enormous park with the theater at the center. The theater stood bare, with a huge chunk of its red roof on the ground. The meter (three-foot) thick walls by the field kitchen had disintegrated to dust.

Her mind froze. Her mother and sister were inside.

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The airstrike hit around 10 a.m., squarely on the stage and field kitchen.

Maria Radionova had laid out a corner for herself and her two dogs just underneath, in the hall of the drama theater with the chandelier. The roof caved in and the chandelier shattered.

Radionova wasn't there. She had gone to stand on the steps at the entrance to the theater.

She heard the telltale whistle from a plane. A man grabbed her by the neck, pressed her against a wall and covered her. Debris and fragments of bricks flew at them.

The explosion threw another man back and face down onto glass. A wounded woman lay nearby in a huge pool of blood.

Radionova went back into the theater and tried to get into the hall. People were running and screaming, and lost children were frantically looking for their mothers. Radionova knew her dogs were dead.

"They were all I had," she said, crying. "This (was) actually my family. … I cried there for probably two hours."

Victoria Dubovytska, 24, had just folded blankets into a pile in the projection room where she was staying with her 2-year-old daughter, Anastasia, and 6-year-old son, Artem. When the bomb hit, they were thrown against the wall. The blankets tumbled on top of the toddler, shielding her small body from the slabs that fell next.

In the first seconds after the shock, the room was silent. Dubovytska feared her daughter was dead. Then Anastasia's voice joined the other screams: "Mama!"

"I understood she was alive," Dubovytska recalled. "I dragged her out….It was a miracle she survived."

She took her son, her daughter and any documents she could find and ran out of the theater. Half of it had already crumbled.

As people fled the opposite way, Maria Kutnyakova ran into the hall looking for her mother and sister. She went to the third floor, but the windows were shattered and there was no sign of her loved ones or their belongings.

Hoarse shouts for family members filled the air. At first she too shouted "Mom," but she quickly realized that everyone around her was shouting the same word. So she screamed the family name instead.

Someone answered, "Masha Kutnyakova!" With everyone shouting, she couldn't figure out where the voice came from. It sounded like it came from somewhere in the ground, but only the dead lay there. She thought she was going crazy.

She went to the stairs down to the basement and bomb shelter. There, at the bottom, stood her sister, covered in plaster dust, with a cat. She had been on the third floor and fled to the basement for cover.

Their mother wasn't upstairs but on the ground floor, near the medic's office, and escaped out of a side exit. They made their way with a crowd of about 50 people to Mariupol's Philharmonic, a nearby auditorium which was also serving as a shelter. That too came under shelling at sunset.

"I wasn't killed in the theater, but I'm going to die in the philharmonic," Maria Kutnyakova told herself bitterly. "God, this is my cultural program for the day."

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The shockwave from the explosion also reverberated outside the theater.

March 16 was Dmitriy Yurin's 31st birthday. He was headed the 100 meters from his home to the theater, as he had done every morning in the past week, for food and water.

Near the entrance to the parking garage, the force of the blast knocked him to the ground. Yurin, a fisherman, picked himself up and ran to help, moving rubble to drag out those who were alive but couldn't walk.

"I looked at my arms, and they were covered in blood up to the elbow," he said. "And I was in a stupor, just shock."

He left for about 20 minutes to collect himself and rub off some of the blood, then returned. Most of the bodies were unreachable deep in the foundations, which were now in flames. Anybody they could reach, rescuers moved to the park.

"Some of them weren't alive, and some of them breathed their last on the street," Yurin remembered, sighing. "We said goodbye to them."

One young woman — maybe 25 years old — stood out in his memory. He stuttered as he recalled her face.

They laid her out on a bare winter flowerbed, still conscious. Two women and a child stood by her, trying to reassure her through their tears.

"We'll live, don't die, everything will be fine," they said. "You'll get help."

But she died in front of him.

Yurin left soon after. He numbly pulled on a neoprene suit he used for fishing on cold winter days and wrapped his feet in plastic bags. Then he plunged into the Azov Sea and swam for nearly a kilometer (half-mile) "like a dog" before emerging outside Mariupol. It took days, but he eventually made his way to safety in western Ukraine.

Yulia Marukhnenko also had been renting an apartment near the theater. When she heard the bang, Marukhnenko first looked to the field kitchen, but she knew everybody there was buried. So she rushed to the basements.

Trained in first aid, with a full kit on hand, she was facing problems no first aid could begin to help: limbs attached to no bodies, bodies with no limbs, bones sticking out. Those were the ones who died, either on the spot or in the days afterwards in a city with almost no functioning hospitals. One woman had her leg amputated but died anyway.

Marukhnenko and the two police officers working alongside her said a dozen people were pulled from the rubble, the last one around 4 p.m., six hours after the airstrike. Her name was Nadia.

Still in shock, Nadia said the explosion pulled her young son and husband away, and they died in the basement. The woman cradled a dachshund that belonged to her son, who had named the puppy Gloria. Nadia begged her rescuers to take the dog.

She asked for a cigarette. She said she hadn't smoked for seven months because her son had asked her to quit. But there was no longer anybody to quit for.

Nadia was taken to the hospital, and Marukhnenko doesn't know what happened to her. The dog is with Marukhnenko still.

"If Nadia has survived, tell her that Gloria is fine," Marukhnenko said. "She's eating well, she's all right, and she's with me."

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The theater now lies in ruins, with its side and center blackened by fire. Russian forces control the neighborhood around it, and AP video shows heavy equipment swarming the rubble to further dismantle it. But the questions remain: How many bodies are there, and what happened to them?

A police officer who passed the theater a week after the airstrike said the smell of death was overpowering. He spoke on condition of anonymity because he still has relatives in Russia-controlled territory. Video taken by Russian state media shows no bodies inside, contrary to the descriptions of multiple witnesses.

The lack of bodies led the police officer and a Mariupol Red Cross official to speculate that perhaps fewer than 500 people died, but most survivors suggested the bodies were either pulverized into the dust or removed by the Russians. With the site off-limits to investigators and the rubble itself taken away, witness testimony and photos and video of the theater before and after it was bombed will be crucial, said Clint Williamson, who served as U.S. ambassador-at-large for war crimes issues from 2006 to 2009.

"Without being able to get to the scene, it is going to be difficult to go much beyond that," he said.

The Organization for Security and Cooperation in Europe has declared the attack on the Mariupol drama theater an "egregious violation" of international humanitarian law. The organization's mid-April report found that "those who ordered or executed it committed a war crime." It also found no dispute that the destruction of the theater was deliberate.

This finding was echoed by two munitions experts interviewed by the AP, who said the scope of the destruction points to a 500-kilogram bomb from a Russian warplane.

"It's much too much for an artillery shell," said Mark Cancian, an explosives analyst at the Center for Strategic and International Studies and a former artillery officer. "The fact that it hit square on would lead me to believe that's what they were aiming at."

Russian troops want to take over Mariupol because of its strategic value as a port and a link between territories in the south and east held by Russia-friendly forces. Moscow has declared victory, but Ukraine refuses to acknowledge defeat.

In the meantime, families are desperate for any news of loved ones. A Telegram channel for Ukraine's missing has more than 19,000 posts, with photos and other details. More than 9,600 refer to Mariupol alone.

The survivors from the theater attack remain haunted by their memories of what the Russians did.

"They came not to capture the city — they came to destroy it," said Maria Kutnyakova, sitting in another auditorium in the city of Lviv where artists recently staged a show to honor Mariupol's theater and those killed inside. "They are trying to hide how many people actually died in Mariupol, hide their crimes."

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Beatrice Dupuy and Marshall Ritzel in New York, and Cara Anna in Lviv, contributed to this report.

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Follow the AP's coverage of the war at https://apnews.com/hub/russia-ukraine