I have the world's most dangerous job - Putin wants me dead and that's just the half of it

As we drive through the Ukrainian countryside in his armoured, unmarked 4X4, Kharkiv police chief Serhii Bolvinov pulls out his phone to show videos of a new hobby he uses to relax between Russian war crime investigations.

"When I have a free day, which unfortunately is not very often, I take my sniper rifle, go to the long-distance range and shoot," he says, pointing to a slow-motion video where he peers through a scope as bullet casings cascade into the air. "The longest distance I've hit the target from is 1.8km," he adds with a smile.

For a man whose days involve interviewing victims of torture and painstakingly chronicling how children were murdered, shooting might sound like an unusual way to let off steam.

But for those living in a war zone, even pastimes tend to have a practical edge, especially if you are, like Serhii, personally wanted dead by Vladimir Putin's regime.

Since the full-scale invasion in 2022, Russian bombs have destroyed multiple Kharkiv police headquarters and targeted crime scene evidence. A warrant for Serhii's arrest has been issued by an occupied territory. He has 1,000 people working for him, and many of his colleagues are being killed in the line of duty. And as Serhii, 42, continues to put himself in the firing line by collecting evidence of Russian war crimes, I've come to the front line to watch him work.

"They published my personal mobile number, information about my family and where I live," he tells me as the car slows for a military checkpoint with sandbags and soldiers.

"It was not just me [targeted] but also my team, who investigate war crimes."

Serhii pauses to pull up the Russian wanted poster on his phone that was sent to thousands of people on the encrypted app Telegram.

To "save his life", he was forced to move house, get rid of his phone and source a car that can withstand gunfire.

All his officers now work from secret, unidentifiable locations and, to avoid being tracked, he takes extra care when using the internet.

The father-of-three has seen with his own eyes how even the most trusted friend can be a traitor who feeds information to the Russians - the result being that innocent people are murdered.

"We found an FSB agent here in the Kharkiv region," he adds, "There are a lot of bad things that Russia does in our territory using collaborators and [secret] agents."

One example that sticks in his mind was when a funeral party in a 300-person village was bombed by Russia after an inaccurate tip from a collaborator that a senior army official was present.

The attack killed 49 people, including children, affecting almost everyone in the area. Serhii later discovered that the man who sent the text that caused the carnage escaped justice by fleeing to Russia.

Living amid such distrust, anxiety and danger would reduce many people to a quivering wreck - and while the police chief's voice does crackle with emotion as he describes encountering some of the most gruesome crimes of Russia's war in Ukraine, most of the time there is barely a line on Serhii's face.

Kharkiv is best known as the site of one of the war's most crucial battles of 2022. Ukrainian forces won a decisive victory there, stopping Putin's westward advance. The city's combination of futuristic Soviet-era architecture and towering piles of rubble often makes me feel like a small cog in a very large machine.

From a distance, people on the endless Freedom Square look like matchsticks, especially beneath the turrets of the Palace of Industry, which resembles the skyline from the dystopian 1920s science fiction film Metropolis.

For Serhii, who's lived in the city since joining the Ukrainian National Police at age 16, Kharkiv is simply "home".

"I was born in a small town that's just under 40 miles from Kharkiv," he says, "but my whole life is tied to [the city]. I was educated here and it gave me a career. It was the place I met my wife and where my three kids were born."

In December 2021, as Russia was amassing troops on the Ukrainian border, Serhii was given the biggest job of his career to date and became the head of investigations for the region. As Putin's war machine began its brutal invasion not long after, Serhii came to realise his responsibilities would be vastly different from those of any police officer in Kharkiv until now.

Of course, it can be the case that police officers face personal danger in peacetime, but this is different.

"We are a target for the Russian military," Serhii adds, "some of my colleagues were killed by ballistic rockets, others have been killed by drones. [Investigating their deaths] is a different and hard situation. But it's a crime and it's our work, so we keep going."

The efforts to bring those who've murdered, tortured and raped civilians to justice never cease in the secret offices of his investigative unit.

Serhii takes us there in his armoured car. Coffee in hand, the police chief navigates his way through the towers of documents filled with testimony and details of physical evidence to chat to his team. He stops to examine the faces of some suspects whose pinned-up photos stare out from concrete walls.

In one picture, a man convicted of killing five civilians is having a medal pressed onto his chest by Vladimir Putin. He is one of the many criminals protected by the Russian regime who Serhii's team of investigators has successfully prosecuted in absentia.

I asked him if it's frustrating to watch a president laud a man as a hero who he knows has committed such awful atrocities.

"I do this for the future," he replies, "I hope one day every Russian criminal will be punished. In my mind, the Russian military who have [committed] war crimes are the same as Nazis from the Second World War."

These sorts of prosecutions adhere to the same standards as with any other crime, although the chaos of war can sometimes make things harder.

Physical evidence and interviews often form the backbone of a case, with new technologies and military intelligence helping to identify suspects.

In his office, Serhii shows me an intricate digital 3-D model of a crime scene being used for an investigation into a mass grave in the town of Izyum discovered in September 2022.

For us to truly understand the scale, he takes us there to see it first-hand.

If it weren't a site of such terrible tragedy, the woods on the edge of Izyum would be peaceful. There is a deep smell of pines and a gentle sweep of the branches moving in the wind.

Staring out at more than 400 graves marked with crude wooden crosses, Serhii recalls the moment he learned of the site.

"After the occupation, civilians in the area told police officers about this place," he says. "My investigators came here, took a photo, shared it with me and the next day we started to work. A big team came down and over seven days, we exhumed 448 bodies."

Most of the remains were in such bad condition that they could only be identified by having relatives of missing people turn up and give DNA samples in large numbers. It is not yet clear why exactly Putin's troops created the mass grave in Izyum. Serhii has established that some of the victims were killed by Russian air strikes and then dumped in the woods, but he hasn't found a reason the bodies had numbered crosses stuck in the ground above them.

Russian propaganda has claimed the mass grave relates to crimes committed by so-called "Ukrainian Nazis".

There is no shred of validity to such claims; there are, however, strong indications that Russia is trying to destroy evidence.

As they retreated, Russian forces filled the areas around the mass civilian graves and pile of soldiers' bodies with mines. Then, when the bodies were exhumed, the refrigerators holding them were hit in what appeared to be a targeted air strike.

"We had five rockets fired at the place where the bodies were. They hit one of the refrigerators and close by," Serhii explains.

He can't conclusively prove the rockets were fired at the bodies, but there were no other plausible targets in the area.

Looking at the burial site, Serhii's voice grows hoarse with emotion. This is a crime scene that traumatised even the most hardened Kharkiv police officers.

"I feel terrible," he says. "This doesn't happen to Putin or his military commanders -they made the decision to start this war, not these civilians; they should be alive. I think of the children who we found here.

"The youngest was a seven-year-old boy, killed by ballistic rockets. He and his family were in a basement, trying to be in a safe place, but Russia killed him."

Serhii is not only fighting for the dead, he's seeking justice for the living. Every day, his team is told of new crimes inflicted on the civilian population during the occupation.

Dusting the brown Izyum dirt from our shoes, we step into the car to visit the venue where such horrors took place, an abandoned police station that became a Russian torture chamber.

Serhii squeezes on a set of plastic gloves and pulls out a flashlight as the heavy metal door creaks open. It's cold even in the relatively mild spring conditions; you can only imagine what the freezing snow of March 2022 would have felt like for the prisoners lying on concrete floors beneath windows made only of metal bars.

"Most of the prisoners in these torture chambers were there because they didn't want to be a part of Russia," he says.

"Russian soldiers caught everybody who appeared pro-Ukrainian. It could have been because they spoke the language, displayed a flag or because a collaborator said they were close to the military. One of the victims was tortured by having a bicycle spoke pushed under the skin - for other people, they used electricity from car batteries.

"I met one man who was here who had his arm broken and was beaten with a bat. We also had some rape cases in the torture chambers."

The stretch of suffering remains overwhelming to the senses. Buckets used as toilets are filled with foul black liquid and litter the corners. There is torn clothing on the mattresses.

Serhii shines his flashlight on the walls to reveal the marks made by a prisoner seeking to count the days. Below, a prayer has been carved into the wall: "God save us, amen."

"It's hard to listen to the stories about this place," says Serhii. "But these are our people and we will continue to fight for future justice. It's important for us and for Ukraine."

Rain has started to fall gently as we part ways. Serhii shakes my hand firmly and steps into his armoured car.

It's unlikely things will get any easier but he knows he must keep going: who else is on Ukraine's front lines digging through rubble for fresh evidence of the atrocities Russia is committing?