As evidence of atrocities in Ukraine continues to emerge, Western political leaders, including President Joe Biden, have said Russia has committed genocide. But legal scholars say the burden of proof for genocide is hard to meet, and the legal community is split as to whether the evidence gathered so far does that.

A crucial element of genocide is a specific intent, known in criminal law as "mens rea," to carry out a systemic destruction of a targeted group.

Patrick J. Keenan, a professor and researcher at the University of Illinois College of Law, said he thinks the evidence that has emerged so far doesn't support an intent to systematically annihilate the Ukrainian population.

"We need more evidence," he said. "What's happening in Ukraine, as tragic as that is, it looks like this is abuse of civilians, which is awful and illegal. But that's what the category of crimes against humanity is for."

"Atrocity crimes" is the umbrella term used to describe genocide, war crimes, crimes against humanity and aggression — all of which have distinct legal definitions, thresholds and political weight.

Genocide presents the highest legal threshold. The Rome Statute of the International Criminal Court says genocide involves killing and other acts "committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group."

Crimes against humanity involve murder, rape, torture, forcible deportation and persecution against a recognizable group "as part of a widespread or systematic attack directed against any civilian population," under the ICC statute.

War crimes are defined as "serious violations of the laws and customs applicable in international armed conflict."

Intentionally attacking civilians and nonmilitary buildings, bombing towns or villages, attacking humanitarian convoys and unlawfully deporting people are war crimes. So is killing or wounding unarmed enemy soldiers who have surrendered.

A fourth type of atrocity, aggression, is defined as the military invasion by a state "against the sovereignty, territorial integrity or political independence of another state."

Since invading Ukraine on Feb. 24, the Russian Army has killed civilians and attacked residential areas, schools and hospitals. Numerous allegations of women being raped have surfaced. Bodies of civilians strewn on roads were found in Bucha and other suburbs of Kyiv. More evidence of extrajudicial killings of civilians has surfaced as the Ukrainian army regained control of towns and villages that were occupied by Russians.

Inna Liniova, the CEO of the Ukrainian Bar Association, told Law360 the evidence supports a claim that Russia is carrying out genocide.

"When we look at formal definitions of genocide, we think that all these actions are being committed in Ukraine," Liniova said.

Statements made by Russian President Vladimir Putin about his plans to "denazify" Ukraine point to an intention to rid the country of anyone who supports an independent Ukraine or doesn't share Russia's interests. Actions by Russian soldiers also suggest genocide, she said.

"We have multiple cases of Russian soldiers destroying history books, Ukrainian history books. We have cases of Russian soldiers shooting people for speaking Ukrainian," Liniova said. "The threshold has been reached."

She added that "ultimately, this is indeed for a court to establish."

Biden said last month he believed Russia committed genocide.

"It sure seems that way to me," he told reporters.

The Biden administration's quickly escalating rhetoric is puzzling legal experts. Declaring a genocide carries heavy political weight, particularly when the accused perpetrator has the geopolitical and military might of Russia, a country of nearly 146 million people, they say.

Paul Williams, a seasoned investigator and co-founder of the Public International Law & Policy Group, a global pro bono firm that investigates atrocity crimes around the world, said meeting the burden for genocide requires extensive research and rigorous legal analysis of the evidence gathered on the ground.

Attorneys and investigators who deal with atrocities are careful to apply the label.

"One reason why there's a reluctance to use the term without this type of systematic investigation, is that it becomes diluted," said Williams, whose firm helped the U.S. State Department rule last month that the Burmese Army had carried out genocide against the Rohingya, a predominantly Muslim ethnic group living in Myanmar.

Williams said extensive evidence-gathering and analysis is required before the U.S. government can declare certain acts as atrocities, in particular crimes against humanity and genocide.

"President Biden and others have used this language much, much, much more quickly than I would have thought," Williams said.

Keenan, of the University of Illinois College of Law, said politicians are afforded "a lot of leeway" in what they say as they try to generate support, but lawyers have to stick to the law.

"I understand why politicians want to label something genocide, especially when this could end up being genocide. So it's not that it's ridiculous to say that. By any means, it's not far-fetched," Keenan said. "But I think as lawyers, we have to be careful. Attaching the right label actually does matter."

Hank Greenberg, a shareholder at Greenberg Traurig LLP and a past president of the New York State Bar Association who recently took a trip to the Polish-Ukrainian border to meet with war refugees, called "premature" any speculation on which charges Russia would be facing if brought before an international court. There is, however, a rapidly emerging consensus that war crimes have been committed, he said.

"There's little doubt that they're targeting civilians. That's no longer up for [debate], certainly after Bucha," Greenberg said.

But Greenberg said he was stunned at how Biden's accusations quickly soared from calling for Putin to be tried in a war crimes tribunal to accusing Russia of carrying out genocide.

"What's stunning to me is the rapidity with which things are moving," he said.

Greenberg said the speed is due in large part to public outrage at images of atrocities that have proliferated on social media.

Since the war began, videos and photos showing artillery attacks on residential areas have abounded, including a maternity hospital and a theater in the southern city of Mariupol, where a large number of casualties have been reported.

Keenan said the massive amount of evidence gathered and submitted by ordinary civilians in Ukraine presents a relatively new scenario in the investigation of atrocities. In the past, that was done mainly by investigators working for prosecutors.

"The fact finding process for Ukraine is fascinating, and will be something that we had never seen before," Keenan said.

The ICC set up a portal for individuals to upload what the law enforcement community calls open-source intelligence or user-generated evidence: stories, images or videos documenting possible atrocities.

The government of Ukraine set up a similar portal. The Ukrainian Bar Association, a voluntary association with about 7,000 members, has created a group of 35 volunteers doing their own open-source investigation.

The NYSBA, which formed a Ukraine task force years ago to promote the legal profession in that country and has a relationship with that bar, is assisting Ukrainian lawyers with the investigation of atrocities by providing advice on how to best preserve evidence collected on the ground.

Another major contribution to investigating atrocities comes from the use of satellite imagery, Keenan said.

"There's nothing objective in any international conflict. But this is fairly close to objective. It's just up there in the sky. It's not politicized, and it's taking pictures of what's happening on the ground," he said. "If there are bodies in one place this week, then the denial that they were actually killed becomes much less credible."

Keenan said prosecutorial investigation, user-submitted evidence and satellite images will "help to tell a very powerful story," with the caveat that prosecutors will have to rely more and more on sophisticated verification techniques to ensure the evidence is credible.

Zachary D. Kaufman, a law professor at the University of Houston Law Center who has served in three war crimes tribunals and was the first American to serve at the International Criminal Court, told Law360 that the evidence currently available likely meets the legal thresholds for several types of atrocities.

In addition to the bodies recovered in Bucha, sites of possible mass graves have been found near the southern city of Mariupol, which has been under Russian siege for weeks.

Earlier in the conflict, Russian soldiers seized the nuclear plant of Chernobyl for a month, attacked humanitarian corridors, and allegedly used banned weapons such as cluster munitions.

On April 8, a Russian missile attack killed 57 people and wounded over 100 at a train station in the eastern city of Krematorsk, where a crowd of civilians was trying to board trains to flee the region.

"Russia appears to have perpetrated war crimes and crimes against humanity through its indiscriminate attack on civilians and civilian objects," he said.

Kaufman said Russia could also be responsible for genocide for its actions such as the ones in Bucha, because they show an intent to kill the civilian population.

Many genocides in history — such as the Holocaust, the Cambodian genocide, and the genocide against the Tutsi in Rwanda — involved mass killings, but jurisprudence has determined that the number of civilians killed is not a threshold in itself.

"It's a common misconception that you need mass murders to qualify for genocide," Kaufman said, adding that the ICC statute "does not specify how many people or what proportion of people you have to kill in order to qualify for genocide."

Whether for genocide or other atrocities, there is also a question of whether the Russian Federation or Putin himself may face charges.

Kaufman said Putin and members of his circle could be tried in the International Criminal Court, the world's first permanent international war crimes tribunal.

The ICC is currently investigating Russia's atrocities in Ukraine. Although neither of the two countries is a member state of the ICC, Ukraine has accepted its jurisdiction. More than 40 countries have urged the court to investigate the situation in Ukraine.

This week, the New York City Bar Association urged Congress and the Biden administration to assist the ICC's investigation.

"We now call on the United States to abandon its prior reluctance to support the ICC so that the escalating pattern of war crimes being carried out in Ukraine can be properly investigated, adjudicated and sanctioned," the City Bar said in a statement.

Putin could also be tried in an ad hoc tribunal such as the ones that presided over the first Nuremberg trial and the Tokyo trial at the end of World War II, the United Nations tribunals for Rwanda and for the former Yugoslavia, and the tribunals for Sierra Leone and Cambodia.

A third venue to prosecute Putin and his allies could be through Russia's domestic courts.

"That is, of course, unrealistic at this time, but it could be possible in a post-Putin world," Kaufman said.

The fourth option is through another country's domestic courts, via a principle called universal jurisdiction, for which sovereign states are able to prosecute certain crimes regardless of where they were committed and regardless of the nationality or residence of the alleged perpetrator.

"Universal jurisdiction is the claim that some offenses are so heinous that they can be prosecuted by any country, anywhere in the world," Kaufman said.

But the concept of universal jurisdiction is controversial, as many countries do not subscribe to it, considering it a breach of sovereignty. The United States and Russia do not recognize it.

Some European countries, including Germany and Sweden, have launched investigations focusing on atrocities in Ukraine. Germany already used the principle to prosecute and convict Syrian officials for acts of torture carried out during the ongoing conflict in Syria. Other countries recognize jurisdiction over a foreign country only if there is a nexus between them.

"Some countries are more strict in their interpretation of their courts' own jurisdiction, and would therefore not hold the view that universal jurisdiction is appropriate, or even legal," Kaufman said.

--Editing by Marygrace Anderson and Emily Kokoll.

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