Investigating war crimes is a complex, horrific and often dangerous task.
For UK barrister Nigel Povoas, the scenes and stories he's come across in Ukraine are like nothing he's seen anywhere else in the world.
"It's pretty devastating as a human to see and hear this, to be honest," he says.
Mr Povoas is the lead prosecutor for the International Mobile Justice Teams, a multi-national group of legal experts supporting the Office of the Prosecutor General of Ukraine.
He's been in the country since June and has spent the past week close to the conflict's frontline in the south, near Kherson.
Mr Povoas told ABC News Daily the evidence he's collecting is devastating.
"People are talking about detention centres, torture and enormously awful sexual violence," he said.
Much of Mr Povoas's work has been investigating missile attacks on civilians, visiting nearly all the major strikes within 24 hours of them taking place.
Recently, his focus has shifted to sexual crimes, allegations of which are now increasingly common in formerly Russian-occupied parts of the country.
"This is part of an increased effort on the part of the prosecutors and investigators in Ukraine to ensure that those who have suffered sexual violence are able to come forward in a safe space, to make their complaints and tell their stories so that justice can be done," he says.
"Stories like women being rounded up in villages and being stood in a line to … be used for the purposes of the Russian soldiers' enjoyment and entertainment.
"Women of all ages, ranging from 13 to 82 … it's pretty devastating."
Mr Povoas says it's important to be on the front line soon after Russian forces have left to demonstrate to the victims that they are not being ignored.
A pattern of atrocities
Several groups, including the United Nations, have concluded that Russia has committed war crimes in Ukraine, but proving this in a court of law is another thing.
There have already been a small number of war crimes trials held in Ukraine with at least three Russian soldiers convicted and sentenced to more than 10 years in prison.
But Mr Povoas's experience is in building cases against senior figures, something he acknowledges is a more complicated task.
"The way to do that in a military situation — unless you can get your hands directly on communication intercepts of people talking about their knowledge — is to show a clear pattern of war crimes which you can then infer must be happening by design."
He believes the repeated incidences of torture, civilian killings and sexual violence do suggest the crimes are not simply the actions of individual Russian soldiers.
"It really does form part of a genuine pattern that's happening across Ukraine in those territories where the Russian forces have recently occupied," he says.
To build this evidence, it's crucial for war crimes investigators to begin collecting evidence as soon as possible after Russian soldiers have left.
This urgency led Mr Povoas to Izium in the country's north-east last month; he visited the day after Ukrainian forces regained control in a significant counter-offensive.
Several mass graves were discovered in the forests surrounding the town, one containing the bodies of more than 440 people, many of them civilians.
"Some of the corpses have their hands tied and clearly have been subject to torture," Mr Povoas said of the bodies discovered there.
"That's not to say that soldiers aren't also tortured, but I think it becomes clear fairly quickly after careful exhumation which is civilian and which is military."
He says the kinds of mass graves discovered in Izium are reminiscent of those left by Russian soldiers in the city of Bucha, near Kyiv, earlier in the conflict.
"It's a very graphic reflection of what's actually happening here and what some of the Russian forces, or many of them are doing," he said.
Targeting civilian infrastructure
In recent weeks, the Russian army has increased it's reliance on another form of violence that could also constitute a war crime — the targeted bombing of civilian areas and infrastructure.
The attacks, launched with missiles and Iranian-made kamikaze drones, followed an explosion on a key bridge linking Kremlin-controlled Crimea with Russia.
Russian President Vladimir Putin has said the bombings, which included attacks on hospitals and a playground, were retaliatory strikes.
"If attempts continue to carry out terrorist acts on our territory, Russia's responses will be harsh and in their scale will correspond to the level of threats created for the Russian Federation," Mr Putin said at the time.
Russia denies it is targeting civilians, but Mr Povoas believes there's strong evidence this is occurring.
"I've been very, very close to the explosions and then seeing the sites the next day, it is terrifying," he said.
"This looks like a very clear and cynical intent to spread terror and cause mass suffering across the civilian population."
At least 38 civilians were killed and more than 100 wounded in the attacks launched against Kyiv, Dnipro and Zaporizhzhia.
Mr Povoas says the Russian tactic of attacking energy facilities and other infrastructure may also be considered a war crime.
"If there is a concrete military advantage, which isn't disproportionate to its impact on the civilian population, that might not be a war crime, but the military advantage appears to be extremely minimal and in some cases, not any at all," he said.
"As the cold weather and the winter closes in, it's going to cause mass suffering and real pain and loss of life as the weeks and months progress."
Building a case right to the Kremlin
Mr Povoas's priority is investigating the culpability of very senior Russian commanders and politicians, a task he says could take several years.
"They are inherently more complex and therefore it will take more time to investigate and also to make the links up the chain of command to the leadership right up to the Kremlin," he says.
"It requires a meticulous approach to ensure as compelling and as cogent a case can be built."
Other war crimes trials, such as that of former Yugoslav president Slobodan Milosevic, have taken many years to come about, and in Milosevic's case, he died in prison before the trial could be completed.
But Mr Povoas says the investigations of Russian leaders for war crimes are occurring at an unprecedented speed.
"I don't think it's happened before where such widespread investigations have taken place across a country still very much at war," he says.
"Whether those senior commanders and senior politicians responsible are actually arrested and brought physically to justice, that remains to be seen.
"We're confident and extremely focused in trying to make that happen."
