The shocking images and stories out of Bucha and surrounding towns in Ukraine have led to calls for a "war crimes trial" against Russia.

Ukraine President Volodymyr Zelenskyy told the United Nations Security Council on Tuesday that it was the "most terrible war crime" since World War II. He said the Russian military killed civilians "just for their pleasures." He accused them of shooting men in the back of their heads and raping and killing women in front of their children.

"Anyone who has given criminal orders and carried them out by killing our people will be brought before the tribunal which should be similar to the Nuremberg tribunals," he said.

Earlier on Monday, United States President Joe Biden had also joined leaders around the world in calling for a war crimes trial against Russian President Vladimir Putin. He said Putin was "a war criminal" who should be brought to trial.

But experts warn the path to holding the Russian president and other leaders criminally responsible is long and complex.

Clint Williamson served as a U.S. diplomat for war crimes issues from 2006 to 2009. He told the Associated Press: "Certainly, the discovery of bodies which bear signs of executions -- such as gunshot wounds to the head -- presents strong evidence of war crimes." He added the actions were not permitted whether those who had been taken prisoner were civilians or soldiers.

Russian officials strongly denied the accusations. Its defense ministry said that "not a single civilian has faced any violent action by the Russian military." And Foreign Minister Sergey Lavrov claimed that the images were "staged" to bring anti-Russian feelings.

The New York Times disproved the Russian claims in its study of satellite images provided by Maxar Technologies. The Times reported that at least 11 bodies of those killed in Bucha had been on the street since March 11. Russian troops did not withdraw from the area until March 30.

War crimes investigation

The International Criminal Court, or ICC, has already opened an investigation in March into crimes against civilians in Ukraine. The investigation came after Russian airstrikes hit a hospital for children and pregnant women in Mariupol.

To build a case for war crimes, prosecutors must gather evidence to establish the cause and facts of the victims' deaths. They also need to show that the crime happened in an armed conflict, which is the case in Ukraine.

Then comes the more difficult work of establishing who is responsible for the crimes. It could be forces on the ground, military commanders or government leaders.

Andreas Schüller is the program director for International Crimes and Accountability at the European Center for Constitutional and Human Rights in Berlin. He said prosecutors need to look into the chain of command and establish a link to the evidence.

"Documents could be leaked, or witnesses could speak up and disclose internal planning operations," he added.

Building a case against Putin and other Russian leaders will be more difficult. "You've got to prove that they knew or they could have known or should have known," said Philippe Sands. He is a British lawyer and professor at University College London. He warned there is a risk that the trials will involve mid-level people without reaching the top leaders.

Crime of aggression

An easier path would be to charge Russian leaders for the crime of aggression, which is the act of launching a war without cause against another country. However, the ICC has no jurisdiction over Russia for the crime of aggression because Russia, like the U.S., has not agreed to the treaty. Russia could also veto any attempt in the U.N. Security Council to bring the case to the ICC.

In March, Ukrainian Foreign Minister Dmytro Kuleba and former British Prime Minister Gordon Brown launched a campaign to create a special tribunal to try Russia for the crime of aggression in Ukraine.

The tribunal will need the backing of an international group like the U.N. or a collection of nations. For example, after World War II, the Nuremberg tribunal was established by the U.S., Britain, France, and the former Soviet Union to try Nazi leaders.

Alex Batesmith served as a U.N. prosecutor in Kosovo and Cambodia and is now teaching at the University of Leeds law school. He said the evidence against Russia is very strong. But he added, "there's no way on earth Putin will surrender to the ICC or be arrested and brought to the ICC..."

Last month, ICC prosecutors sought to arrest three Russian generals for possible war crimes in the 2008 war against neighboring Georgia. They have not surrendered so far.

I'm Susan Shand. And I'm Jonathan Evans.

The Associated Press reported this story. Hai Do adapted the story for Learning English with additional sources.

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tribunal – n. a court or forum of justice

stage – v. to create a false scenario

prosecutor – n. one who is in charge of the process of pursuing formal charges against an offender to final judgment

disclose – v. to make known or public

jurisdiction – n. the power, right, or authority to interpret and apply the law