Video journalist Jessica Koscielniak was just getting back to her hotel room in Kharkiv, a city in far eastern Ukraine, when she heard the explosion.

Within minutes, international correspondent Kim Hjelmgaard, in a nearby room, got a call from the city's top prosecutor.

Russia had just bombed a civilian apartment complex. Investigators were rushing to the scene to gather evidence of a possible war crime.

His 84-year-old widow, Anna Satanovskaya, followed as the medics carried his body to the ambulance and placed it inside.

Hjelmgaard and Koscielniak grabbed their vests, helmets and gear and started out. Ambulances streaked past them, going the other way.

Then they saw him. Alexander Satanovskiy, 82, lay bleeding next to a wooden table in an overgrown playground where he had been playing dominoes.

She sat on a barrier outside the doors, crying. Five people had died.

Hjelmgaard explains that war crimes are violations of the laws of war as codified by international humanitarian treaties..

They include atrocities against people or property, murder, ill-treatment, sexual violence, forced deportations, hostage killing, torture, plunder or destruction of public property, and devastation not justified by military necessity.