The first Russian soldier standing trial for war crimes committed in Ukraine apologized to the widow of the man he apparently gunned down.

Vadim Shishimarin, 21, pleaded guilty on Wednesday to the killing of an unarmed 62-year-old Ukrainian man in late February in a village in the Sumy region of northeastern Ukraine.

"Yes, I acknowledge my fault. I understand that you will not be able to forgive me, but I am sorry," Shishimarin said in the courtroom on Thursday, according to CNN.

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There are more than 11,000 war crime cases under investigation, according to Ukrainian Prosecutor General Iryna Venediktova. Last month, her office released the names of 10 Russian soldiers who allegedly committed war crimes, saying the soldiers took “unarmed civilians hostage, killed them with hunger and thirst, [and] held them on their knees with tied hands and closed eyes, mocked and beaten."

The atrocities committed by Russian forces in Ukraine have mostly been uncovered only after they have retreated from the area. The Russians sought to capture the capital city of Kyiv at the beginning of their military operation, and although they failed, they were able to occupy towns in the suburbs.

Once they retreated, horrific scenes were uncovered in towns such as Bucha, where mass graves filled with women and children were discovered, in addition to signs that civilians had been tortured and killed.

The United States and many Western countries have accused Russian forces of committing war crimes, though it will be hard for Ukraine or the International Criminal Court to prosecute the Kremlin leaders who give the orders.

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"If they don't leave Russia, they enjoy impunity for the end of their days. And so, you know, that's the reality of the situation. There is no international police force who can go and cross sovereign territory and arrest them," Beth Van Schaack, the U.S. ambassador at large for global criminal justice, told lawmakers earlier this month. "However, you know, those of us in this business are playing a long game, and there will be jurisdiction over these individuals virtually anywhere they would go because so many states have incorporated within their domestic penal codes the ability to prosecute war crimes and crimes against humanity, regardless of the nationality of the perpetrator or the place of commission."

Members of Congress introduced legislation on Thursday that would expand the U.S.'s ability to prosecute alleged war criminals.