WASHINGTON • The images and reports of human suffering in Ukraine are shocking: destroyed homes and apartment buildings, bodies of bound and shot people littering the streets, women holding infants crawling over the rubble of a shelled maternity hospital, women and children raped, in some cases repeatedly.

Circumstantial and direct evidence as well as eyewitness accounts point to the atrocities being perpetrated by members of the Russian military since it invaded Ukraine Feb. 24.

Ukrainian officials, including President Volodymyr Zelenskyy, global political leaders and human rights advocates have decried such actions, saying international humanitarian law has been violated and calling for those responsible to be held accountable.

Despite the events and daily grisly discoveries, holding the perpetrators accountable for war crimes in a tribunal or the International Criminal Court will be a difficult and complicated task that could take years to unfold, experts in international and human rights law said.

"(Investigations) take a long time because the evidentiary and judicial requirements are very, very high and they are extremely complex cases," said Ernesto Verdeja, executive director of the Institute for the Study of Genocide and associate professor of law at the University of Notre Dame.

The work requires identifying every possible piece of evidence to document each individual incident, Verdeja told Catholic News Service, and even then, a seemingly ironclad case can be difficult to prove.

"It entails not only numerous interviews, it entails trying to gather documentary evidence. It entails trying to understand chains of command and leadership. It entails visiting sites of atrocities and massacres and piecing things together where the information is often at best unclear or at least ambiguous," he said.

Shelley Inglis, executive director of the Human Rights Center at the University of Dayton, acknowledged that investigators should have plenty of evidence with which to work. Average citizens armed with smartphones and journalists on the grounds have recorded apparent international law violations for weeks.

It will be important to interview witnesses as soon as possible while memories are fresh, Inglis said.

Still, gathering concrete evidence is just the first step required to prove that war crimes have been committed. Discovering the military chain of command, who gave what orders and who acted on their own will be almost impossible to determine, Inglis said.

"One of the big issues here Russia is not going to cooperate and they (investigators) will have a barrier in terms what they can prove," she explained.

Steps to gather evidence began within days of the Russian invasion as civilian entities were attacked -- something that international humanitarian law is meant to stop.

Already, the Organization for Security and Cooperation in Europe determined in a fact-finding report released April 13 that Russia broke those laws by deliberately targeting civilians in attacks on the maternity hospital and a theater where hundreds of people took shelter in the besieged city of Mariupol.

Ukrainian officials reported discovering evidence of people being tortured, dismembered and shot at close range in Bucha, a Kyiv suburb. The alleged events led to the suspension of Russia from the U.N. Human Rights Council. Russia has responded that the deaths were "staged" and "fake."

In response to the reports, the International Criminal Court, Canada, Greece and other countries as well as Ukrainian Prosecutor General Iryna Venediktova have opened war crimes investigations. President Joe Biden, despite declaring that he believes Russia has carried out genocide, is weighing how to cooperate with the ICC.

The road for U.S. involvement with the court is uncertain. Concerned that Americans could be prosecuted, Congress enacted two laws in 1999 and 2002 limiting the government's ability to work with the court.

Also, as a matter of policy, the U.S. has always objected to the court's authority in prosecuting anyone from countries that are not part of the Rome Statute, a 2002 treaty that established and governs the ICC's authority.

The U.S, Russia and Ukraine, among other nations, have not joined the treaty. However, Ukraine has twice agreed to accept the court's jurisdiction over alleged crimes under the treaty that have occurred since 2014, coinciding with the Russian occupation of Crimea and its backing of Russian separatists in Donbas in the eastern part of the country.

Centuries before the ICC's establishment two decades ago, prohibitions on certain behavior in war were developed. Such prohibitions on military entities became formalized in the Hague Conventions adopted in 1899 and 1907.

Other agreements focused on the protection of civilians including the Geneva Conventions of 1949 following World War II and two additional protocols adopted in 1977.

Although inquiries are underway, Mary Ellen O'Connell, professor of law at the University of Notre Dame, called for a negotiated end to the war -- the sooner the better.

"I'm concerned that people, in focusing on war crimes investigations, will take their eye off what is the major issue and that's ending the war," O'Connell told CNS.

"As important as accountability is and preserving respect for human rights and hopefully deterring future violations of this kind, we don't have good evidence from the 20 years, 30 years almost, of war crimes investigations in the new era (of the ICC), of actually deterring war crimes or ending conflicts as a result of them," she explained.

O'Connell cited the example of Syrian President Bashar Assad, who has had war crimes charged filed against him in the ICC. "Why would he surrender? He has nothing to live for (if he does)," she said.

She suggested that the United States focus on helping Ukraine craft a negotiating position "that will bring Russians to yes"

"If you charge people with war crimes, call Putin a butcher, you might end up closing the door to the peace negotiations, which have to occur if you're going to have enduring peace," she said.

Long-time peace advocate Joseph A. Camilleri, professor emeritus of international relations at La Trobe University in Melbourne, Australia, agreed that negotiations to end the conflict are a necessary first step on the long road to accountability.

He believes the war is playing out as a conflict between Russia and the United States and that "Ukraine is the proxy on both sides, just like Vietnam was a proxy for the United States, China, Russia many years ago."

Negotiations for peace must allow both Ukraine and Russia to achieve their goals, he said.

"The Ukrainians would gain a cease-fire, which would limit the advances that Russia can make. But there would be limitation on what Russia would hope to achieve. On the other hand (Russia) must agree to negotiate in good faith on all of the legitimate grievances that Russia might have," said Camilleri, a leader of Pax Christi Australia.

He also noted that war crime tribunals of the last 20 years have involved small, less powerful countries, such as Yugoslavia and Rwanda, and he questioned whether a case would ever be filed against the Russian military or political hierarchy.

"There has not been a single case yet of anyone brought to trial from any of the five major United Nations Security Council permanent members or the five major nuclear weapons countries. And I don't think as of yet there is a mechanism to make that happen," Camilleri told CNS.

"Really, I think what the Ukrainian people would most want to see now is an end to the fighting and a gradual return to their home country and an end to internal displacement," he said.

"Then (comes) massive humanitarian assistance. Governments in the West have been generous in their offers of military aid, particularly the United States," Camilleri said. "The economic and humanitarian assistance is nowhere near as impressive and that will have to go into the billions and billions of dollars for the reconstruction of Ukraine."