It took Vadym Bobryntsev four days to bury his wife, he said. It was the middle of March in Mala Rohan, a village near Kharkiv in eastern Ukraine, and the ground was frozen. The village was under occupation by the Russian army, so Vadym went out periodically from their shell-damaged house to dig in a far corner of the garden shaded by apricot and quince trees. After 45 years of marriage, 69-year-old Vadym, alone, lifted his wife Iryna into a makeshift casket, folded her hands over her chest, lowered her into the grave, covered the casket with a corrugated tin sheet, to protect her, and filled in her grave with earth.

Two and a half months later, in early June, a local prosecutor from the city of Kharkiv drove out to Mala Rohan with a small team to dig Iryna up. First, the team walked around and through the remains of Vadym's house, interviewing him and taking pictures of the place where the shell had hit. After the inspection of the house, Vadym led the prosecutor along a path through the garden to the grave. The team took photographs, soil samples, and detailed notes, and watched attentively as two volunteers began to dig. Now the sun was beating down on Mala Rohan and the earth came loose easily. When the casket was raised up and ready to be opened, Vadym, holding a picture of his wife pressed between his palms, ducked under the branches of the apricot tree and walked toward the house so he would not see.

The local prosecutor, Maksym Klymovets, was at Vadym's house to help determine whether the Bobryntsevs were the victims of a war crime - a specialist field in which Klymovets and his colleagues had no experience until Russia invaded Ukraine, three months ago. Klymovets, who has cropped dark hair and a neatly-trimmed beard, was accompanied by two forensic police officers, another officer who took notes, two civilian witnesses drawn from the village - in accordance with Ukrainian law - and two local volunteers to dig the grave. "This is totally new work for us," Klymovets said. "We are learning on the job."

After about three hours, Klymovets declared the evidence-gathering complete for this visit and Iryna's casket was loaded into the back of a van to be transported to the morgue. Her death would be recorded as an alleged war crime - one of nearly 16,000 such cases now open across the country, including more than 1,000 in the Kharkiv region.

There is no precedent for what is happening now in Ukraine - a large-scale war crimes investigation being conducted as the war is going on, by the nation under attack. In Kharkiv, ordinary local prosecutors have found themselves thrust into complex investigations for which they have little or no experience. In the field, they wear vests, some hand-painted, which say "War Crimes Prosecutor", though this job does not technically exist. They are working tirelessly to examine and catalogue scenes of shellings, missile strikes, and murders, and sometimes scenes of brutal crimes against their countrymen.

The work is slow and methodical. Klymovets' team analysed the scene in Mala Rohan carefully, and they will return. Back in the city, in an apartment completely wrecked by a shell, another team measured and photographed every piece of damage, down to a small piece of glass embedded in a door. On the outskirts of a different village, yet another team went classroom to classroom in an agricultural college, tracing the path of a shell that had smashed through each dividing wall without exploding. As they go, the prosecutors narrate their observations in fine detail for the benefit of a video camera. They are following freshly-issued guidelines, and they work with the attentive diligence of people who are new to a task.

The Ukrainians are being supported by some international expertise - explosives technicians from Slovakia; forensics experts from France; war crimes experts from the UK; a 42-member team from the International Criminal Court. But the sheer number of war crimes investigations now open means the vast majority are being carried out by Ukrainian teams. In Kharkiv, before the invasion, there were only 23 people qualified to investigate war crimes, so the region had no choice but to delegate the power to ordinary prosecutors. "Everyone is involved to the maximum," Klymovets said. "We are working seven days a week."

But running war crimes investigations during a war presents significant difficulties. There are complex criteria which have to be determined to assess whether a war crime has been committed. The death of a civilian or destruction of a residential building alone is not proof. Were there Ukrainian military present at the site or nearby? Was the intention to strike a civilian target or was it a mistake? Was it definitely a Russian shell that fell on this building?

Obtaining accurate information about Ukrainian military movements can be difficult, because members of the public are often reluctant to say anything, even to prosecutors, about the locations of Ukrainian troops. The prosecutors try to check with the military, "but we cannot guarantee they will tell us the truth," said Oleksandr Filchakov, the head of the Kharkiv Regional Prosecutor's Office, "because there is a war on after all." Intentionality can be fiendishly difficult to prove, even with the best resources at your disposal. Satellite imagery can assist with determining where a rocket or artillery strike originated, but Ukrainian prosecutors have been unable to access it, because the priority is to use it for military purposes, because there is a war on.

So all strikes on residential buildings and those that resulted in the deaths of civilians are all being recorded as alleged war crimes, under Article 438 of the Ukrainian criminal code. "To be a lawful strike it has to be on a military facility, military unit or some other military object," said Serhiy Shevtsov, a local prosecutor in the Kyivskyi district of Kharkiv, as he examined an apartment building in his district that had been hit by a shell. "If it's a residential block, then it does not matter even if there were military in there until a few days before," he said, "it is Article 438."

This is the prevailing view. But there is concern among some prosecutors and observers that the rush to investigate, while country is still at war, and to brand all these cases initially as war crimes, could undermine a broader future effort to hold Russia accountable.

"It is absolutely fine to be talking about potential war crimes, but the moment you set things in stone you make it easier for the other side to discredit your work," said Philippe Sands, a UCL law professor who has served at the UN International Court of Justice and the European Court of Human Rights. "If you get to a court of law and you can't prove what you said happened, you have only created new problems," he said.

One regional prosecutor concerned with the process is Denys Masliy, the deputy head of the criminal supervisory department, who said he feared there was a resistance setting in to characterising strikes against apparent civilian targets as anything other than a war crime.

"Any prosecutor, any judge in Ukraine, in this environment, who would examine the details of Russian actions and rule they were lawful, he would look like a betrayer," Masliy said, standing under the ruins of a block of flats in central Kharkiv. "As a prosecutor, I must be honest, but this could be perceived as if I am not acting for Ukraine," he said.

Masliy, a tall, good-humoured prosecutor with 24 years on the job, is something of a lone voice among his colleagues. "No one wants to talk about this, it is not a popular topic of conversation," he said. "But after this is all over, someone really impartial could take a deep look at all these cases and find problems. They will say we are fraudsters, our work is fake and we are the same as the Russians. The Russians will use it against us."

Part of the problem stems from a genuine grey area around what constitutes a war crime. A week before the invasion, the regional prosecutors took an online training course on the issue. Masliy posed a scenario to the trainers from the prosecutor general's office: if he took his personal rifle and fired from his window at a Russian tank, and in response the Russians flattened his building, would it be a war crime?

"One of the trainers said, you have a right to shoot because they are invaders," said Masliy. "The other guy said it could be considered a provocation, but he asked, is the danger from my rifle significant enough to justify blasting the whole block of flats? And they ended up having a quarrel with each other."

Three months on from the training session, that hypothetical scenario has become a grim reality. Russia has flattened blocks of flats - with less provocation than a rifle shot - and much worse. The liberated suburbs west of Kyiv revealed execution-style killings and reports of brutal sexual violence. The horrors of Mariupol are yet to be fully discovered. "It looks clear that war crimes are being committed and it looks like they are widespread and systematic," said Sands, the UCL law professor.

The chances of justice ever being served for those crimes, at the higher levels of the Russian army and political system, are remote. The key task now, said Sands, was to gather evidence in the most rigorous way possible. And the key challenge facing Ukraine's newly-minted regional war crimes prosecutors, and its politicians, he said, was to show caution and restraint.

"It is very difficult to show restraint in a time of war, and the people investigating are doing tremendous work, but I would urge real cool-headedness in the characterisation of what is being found and how it is being publicised," he said.

The shelling of Vadym Bobryntsev's house in Mala Rohan, and the death of his wife, was recorded as a war crime, but expert analysis will have to be brought to bear on the case - on small shell fragments from the scene, the likely trajectory of the projectile, on troop movements in the area - before the truth can be established. When that expertise is available is an unknown - there is a severe shortage against the vast number of cases.

When the shell hit the house, the village was occupied by the Russians, raising a question over whether the Russian army would have been shelling its own positions, two weeks into its occupation. This was the prosecutor's initial conclusion - Russian on Russian shelling - but it is possible Klymovets could have to contemplate reclassifying the case under a different legal statute if the evidence suggests it was a Ukrainian shell. "Of course, if we discover that this was done by our military, we will re-classify it," he said.

To Vadym and Iryna Bobaryntsev's son, Pavlo, the important thing was to have "an objective investigation, objective truth," he said. "Truth must always prevail. This is a job that, although not very pleasant, helps to find the truth."

In some cases, for some relatives, the objective truth could mean a verdict that the strike that killed their innocent loved one was lawful, under the rules of war. Pavlo stood alone by his father's vegetable patch, after his mother's casket had been loaded into the van, holding the picture of her he had found among the ruins of the house. "She gave life to me, helped me to solve many problems, treated me with understanding," he said. "How could this not be a crime?"