As more than 2 million refugees from Ukraine begin to scatter throughout Europe and beyond, some are carrying valuable witness evidence to build a case for potential war crimes.

More and more, the people who are turning up at border crossings are survivors who have fled some of the cities hardest hit by Russian forces.

“It was very eerie,” said Ihor Diekov, one of the many people who crossed the Irpin river outside Kyiv on the slippery wooden planks of a makeshift bridge after Ukrainians blew up the concrete span to slow the Russian advance.

He heard gunshots as he crossed and saw corpses along the road.

“The Russians promised to provide a (humanitarian) corridor which they did not comply with. They were shooting civilians,” he said. “That’s absolutely true. I witnessed it. People were scared.”

Such testimonies will increasingly reach the world in the coming days as more people flow along fragile humanitarian corridors.

Ukrainian President Volodymyr Zelenskyy on Wednesday said three such corridors were operating from bombarded areas and, in all, about 35,000 people got out. People left Sumy, in the northeast near the Russian border; the suburbs of Kyiv; and Enerhodar, the southern town where Russian forces took over a large nuclear plant.

“Yes, I saw corpses of civilians,” said Ilya Ivanov, who reached Poland after fleeing a village outside Sumy where Russian forces rolled through. “They shoot at civilians with machine guns.”

More evacuations were announced Thursday as desperate residents sought to leave cities where food, water, medicines and other essentials were running out.

In a staggering measure of displacement, Kyiv Mayor Vitali Klitschko on Thursday said about 2 million people, or “every second person” among the capital’s residents, have left the metro area.

In addition to the growing number of refugees, at least 1 million people have been displaced within Ukraine, International Organization for Migration director general Antonio Vitorino told reporters. The scale of the humanitarian crisis is so extreme that the “worst-case scenario” in the IOM’s contingency planning has already been surpassed, he said.

Russian- and Ukrainian-speaking trained psychologists are badly needed, Vitorino said, as more traumatized witnesses join those fleeing.