Investigators in Poland are collecting testimonies from refugees who fled there to feed to Ukrainian prosecutors. France has sent mobile DNA analysis teams to embed with the Ukrainian authorities to collect evidence. Nongovernmental organizations based in Kyiv, Ukraine’s capital, are going to territories recently occupied by Russian soldiers to collect witness statements.

The involvement of multiple countries and organizations does not necessarily lead to a more productive investigation, said Wayne Jordash, a British criminal lawyer who lives in Ukraine. Mr. Jordash, who is part of an international task force supporting Ukrainian prosecutors, was critical of some of the efforts to assist Ukraine judicially, describing it as “smoke and mirrors,” without results and clear priorities.

The International Criminal Court’s investigators were only just getting going, he noted, and experts from other countries have also been cycling in for stints of several weeks.

“You can’t just parachute into an investigation for two weeks and expect it to be meaningful,” Mr. Jordash said.

Iva Vukusic, a scholar of post-conflict justice at the University of Utrecht, said, “Resources are being poured in, but maybe down the line we will see that they were not being spent the right way,” for instance, duplicating investigation efforts rather than providing psychosocial support to victims.

Ms. Vukusic pointed out the large size of the endeavor. Across the country, she said, “there are thousands of potential suspects, and thousands of potential trials.” All of the material needs to be properly marshaled and analyzed, she said.

“If you have 100,000 items — videos, statements, documents — if you don’t know what you’re sitting on, it limits the use of material,” Ms. Vukusic said.

She also cautioned that the International Criminal Court’s leadership could face criticism by collaborating too closely with the Ukrainian authorities because, she said, Ukraine was also “an actor in this war.”

She feared Ukrainian officials were setting expectations for justice very high, and possibly wasting scarce resources on absentia trials.

“No big case is going to be finished in two years or five years because of the scale of the violence and the fact it is going on for so long,” she said.

Mr. Belousov, the Ukrainian war crimes prosecutor, acknowledged as much. “We are playing a long game,” he said. Even if the perpetrator is tried and convicted in absentia, Mr. Belousov said, “We understand in a year, or two or three or five, these guys won’t be able to avoid punishment.”

Mr. Belousov said that he appreciated the international assistance but that coordinating it was the “biggest challenge” law enforcement authorities experienced.

For example, the Kharkiv prosecutors used a shiny new forensic investigation kit donated by the European Union for their exhumation in Koropy, the village in northeast Ukraine. But a police officer from a unit in Dmytrivka, a 45-minute drive west of Kyiv, said they had not seen or met with any international investigators or received any equipment from them.

Mr. Belousov said Ukraine wanted to take the lead in prosecuting the cases — a divergence from previous post-conflict situations in which the national authorities initially left the process to international tribunals.

But most Ukrainian investigators have little experience in these kinds of inquiries.

For example, Andriy Andriychuk, who joined the police force in the region west of Kyiv two years ago, said his work previously involved investigating local disputes or livestock theft. Now it involves “a lot more corpses,” he said.

On a recent sunny afternoon, he was called to a wooded area near the town of Dmytrivka. Several days before, police officers had received a call from foresters who had come upon a man’s grave. The dead man, Mykola Medvid, 56, had been buried with his passport; his hat was hung on top of a cross made out of sticks.

His daughter and his cousin identified his body. The local morgue officially established the cause of death: a fatal shot in the chest.

Since then, his daughter Mariia Tremalo has not heard from the investigators. No witnesses have come forward, and it was unclear who might have killed her father, or why. Still, she is hungry for justice.

“My father will never be returned,” she said. “But I would like the perpetrators to be punished.”

Right now that seems all but impossible.

In Koropy, the village near Kharkiv, Mr. Ketler’s mother, Nadezhda Ketler, was inconsolable as the gravediggers and inspectors worked. She wandered down the road to another part of her property. Six officials stood over her son’s body, photographing and documenting as his best friend, Mykhailo Mykhailenko, who looked petrified and smelled of stale alcohol, identified him.

The next day, Mr. Ketler’s body was taken to the city’s morgue, where the final cause of death was established.

Eventually, Ms. Ketler gathered the strength to show investigators the crater made by the bomb that killed him, leading the police to the exact spot where he died. Ms. Ketler stood looking at the trees as they rustled in the wind. She did not speak to anyone. She said she did not know if a guilty verdict in a war crimes trial, if it ever came, would ease the pain of losing her child.

“I had to bury my son twice,” Ms. Ketler said later. “You understand, this is hard enough to do once, and to have to do it a second time. The pain of a mother will not go anywhere.”

Evelina Riabenko, Diana Poladova and Oleksandr Chubko contributed reporting.