The bombing of a theater in Mariupol, Ukraine, where hundreds of civilians were sheltering in the early weeks of Russia's invasion, amounted to a "clear war crime," Amnesty International said in a report released on Thursday. 

The human rights group interviewed dozens of survivors of the attack and analyzed satellite imagery, radar data, architectural blueprints of the structure, and video evidence in coming to its conclusion. 

"Many people were injured and killed in this merciless attack. Their deaths were likely caused by Russian forces deliberately targeting Ukrainian civilians," Amnesty International Secretary General Agnès Callamard said in a statement on Thursday. 

"The International Criminal Court, and all others with jurisdiction over crimes committed during this conflict, must investigate this attack as a war crime. All those responsible must be held accountable for causing such death and destruction."

The drama theater in Mariupol, which is a key port city on the coast of the Sea of Azov, became a meeting point for civilians who were fleeing violence and seeking food, water, and shelter. 

On the morning of March 16, Amnesty International believes that two 500kg bombs were dropped on the theater by Russian tactical fighter aircraft

"There was no legitimate military objective proximate to the [theater]. There was no significant military presence inside or close to the [theater] at the time of the attack, or during the days before the attack," Amnesty International said in its statement. 

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"CHILDREN" had been written in large Russian letters on the ground outside the theater for at least three days leading up the attack. 

Russia claimed that its forces captured Mariupol in late May after about two months of intense fighting. 

Estimates of deaths vary widely. Amnesty International says at least a dozen people died, but the actual figure is likely much higher. The Mariupol City Council said 300 died, while an Associated Press investigation concluded that as many as 600 people perished. 

Russia's defense ministry has denied that their forces struck the theater, instead accusing the Ukrainian army of blowing up the structure. 

The Russian Embassy in Washington, D.C., did not return a request for comment on Amnesty International's report on Thursday. 

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The International Criminal Court and UN Human Rights Council opened investigations into alleged Russian war crimes in May. 

Attorney General Merrick Garland visited Ukraine last week and reaffirmed the U.S.'s support in a meeting with Ukrainian Prosecutor General Iryna Venediktova. 

"America – and the world – has seen the many horrific images and read the heart-wrenching accounts of brutality and death that have resulted from Russia’s unjust invasion of Ukraine," Garland said in a statement. "There is no hiding place for war criminals."