The discovery this week of hundreds of bodies buried in a forest near the northeastern Ukrainian city of Izium has cast a renewed spotlight on potential war crimes and prompted fresh calls to hold Russia accountable for any abuses committed during their occupation of the city.
Investigators say the discoveries recall the broad evidence of atrocities by Russian soldiers in towns like Bucha, near Kyiv. But many of the bodies have not been identified, and the causes of death, or even how many were civilians and how many were soldiers, are not yet known.
While the work to clarify how the deaths occurred in Izium continued, Antony J. Blinken, the U.S. secretary of state, said on Friday that it was vital to push for legal accountability.
At a news conference in Washington, he said it was “important that even as the Ukrainians do everything they can to take back the land that’s been seized from them by Russia in this aggression, that at the same time we’re all working to build the evidence and document the atrocities that have been committed.”
“And in many instances, these will amount to war crimes,” he added.
Indiscriminate attacks on Ukrainian civilians have become a hallmark of Russia’s invasion, among them devastating strikes on hospitals, private residences and other targets that have killed and injured thousands.
After Russian forces withdrew from Bucha in April, they left signs of atrocities in their wake.
Investigators building cases for war crimes face immense challenges. More than six months into the war, there are as many as 20,000 continuing war crimes investigations, with multiple countries and international agencies at work, and a high burden of proof to reach a conviction.
In his nightly address on Friday, Volodymyr Zelensky, Ukraine’s president, repeated some of what had been found in Izium and said there was “evidence of torture, humiliating treatment of people.”
“The world must react to all this,” he said.
Next week, he will have the attention of the world’s leaders. The United Nations General Assembly voted on Friday to let him deliver a prerecorded address to the gathering of world leaders in New York, making an exception to its requirement that all leaders speak in person.
A war crime is an act committed during armed conflict that violates international humanitarian laws designed to protect civilians. The rules of war are codified in various treaties, including the Geneva Conventions of 1949 and The Hague Conventions of 1899 and 1907.
Complicating efforts to prosecute potential war crimes is that investigators are working while the war is still raging. The Kremlin has denied allegations against its forces, and Russia’s Defense Ministry has called graphic evidence of atrocities “fake.”
At The Hague in July, representatives from 45 nations, including the United States and European Union countries, heard testimony about atrocities and pledged about $20 million to assist the International Criminal Court, Ukraine’s prosecutor general and efforts by the United Nations.
Experts say the I.C.C., established in 1998 to handle cases of mass atrocities, could be an important avenue for accountability for Russia, though there are obstacles. Neither Russia nor Ukraine is among the court’s 123 member nations, but Ukraine has granted the court jurisdiction over crimes committed in its territory.
Potential war crimes are investigated as any suspected criminal activity would be, through interviewing witnesses, reviewing photos and videos, and collecting forensic evidence, including through ballistics analysis, autopsies and DNA testing. Prosecutors need to prove beyond a reasonable doubt that people knowingly committed the crimes.
Tougher to prove is how much heads of state knew about or were directly responsible for what happened under their command. The history of war crimes cases suggests prosecutors face a formidable challenge to holding Russia’s president, Vladimir V. Putin, to account.
Three of the most prominent prosecutions in history — against Slobodan Milosevic of Yugoslavia, Charles Taylor of Liberia and Saddam Hussein of Iraq — were brought against leaders who were out of power; no sitting president has ever been handed over to an international court.