KOROPY, Ukraine — Four men tugged at long strips of fabric to lift a coffin out of the gaping hole in the backyard of a small house. They flung the lid open to reveal the moldy corpse of Oleksiy Ketler, who had been killed instantly by shrapnel when a mortar fell on the road in Koropy, a village outside Khavkiv in northeastern Ukraine, in March.
Mr. Ketler, a father of two young children, would have celebrated his 33rd birthday on June 25, if he had not been outside his house at the wrong time. Now, his body has become another exhibit in Ukraine’s wide-ranging effort to collect evidence to prosecute Russia and its military for war crimes in the brutal killings of Ukrainian civilians.
Experts say the process is proceeding with extraordinary speed and may become the biggest effort in history to hold war criminals to account. But it faces an array of formidable challenges.
rape, execution-style killings and the deportation of what Mr. Belousov said could be tens of thousands of Ukrainians to Russia — were being investigated.
Better Understand the Russia-Ukraine War
At the same time, hundreds of international experts, investigators and prosecutors have descended on Ukraine from an alphabet soup of international agencies.
Early in the war, the top prosecutor of the International Criminal Court, Karim Khan, arrived in Ukraine with several dozen investigators. But the court, which is based in the Netherlands, tries a limited number of cases, and usually seeks to prosecute only the upper echelon of political and military leaders.
It is also slow: Investigators working on the 2008 Russian-Georgian war did not apply for arrest warrants until this year.
There are a number of other initiatives, too. Amal Clooney, an international human rights lawyer, is part of a team advising the Ukrainian government on bringing international legal action against Russia. The United Nations has started a commission to investigate human rights violations in Ukraine — with three human rights experts — but cannot establish a formal tribunal because Russia wields veto power on the U.N. Security Council.