Even under the impression of the most serious acts of violence, the German senior public prosecutor Klaus Hoffmann is reluctant to jump to conclusions about Russian war crimes in Ukraine. "Our approach is always: to find out broadly and openly what happened. Also to document exactly how which civilians were captured, raped and killed in certain places," says the 49-year-old of the German Press Agency in Kiev.

The lawyer from Freiburg has been advising Ukrainians on investigations into war crimes since the summer. After almost a year of war, the government in Kiev put their number at 70,000. The images of the corpses in the capital's suburb of Bucha, their hands tied behind their backs, and other bloody crimes went around the world. In the meantime, Ukraine has also set up a website that bears witness to the destruction and the suffering of the civilian population. Victims also have their say. Witnesses can report.

The lawyer Hoffmann, who travels from Baden to the Ukraine again and again, sees "a lot of evidence" in the Butscha case - also because the Russians left the place head over heels in the spring. Often, however, it is not easy to prove war crimes - that is, crimes that are not about military goals, but about civilian victims. The United Nations has registered more than 7,000 dead civilians in the war so far. The actual number is probably higher.

Uniform questionnaires help with the investigations

Hoffmann, a member of an international group of experts, helped to create a standardized catalog of questions for the interrogation of prisoners of war. Investigators not only want to find out "why and how someone may have shot a civilian, but also to ask about the background, about the command structures, when and with whom they came to Ukraine."

Above all, it's about fathoming chains of command in order to find those responsible and clarify guilt - right down to Kremlin chief Vladimir Putin. Hoffmann: "In the end, the question is: How can Putin or his defense minister, the chief of staff, the top general or at least one level below be brought to justice for certain crimes?"

It's not easy. This is also shown by the ARD documentary "Charges against Putin? - the traces of war crimes in Ukraine". There, the author Christian Hans Schulz traces how experts from the human rights organization Human Rights Watch (HRW) use computer simulations, satellite images and testimonies to investigate rocket attacks.

After the withdrawal of Russian troops from the city of Izyum in eastern Ukraine, which was occupied from April to September, it became known that dozens of civilians were killed in a high-rise building hit by a Russian missile. Torture cellars were also discovered in the city. Hundreds of bodies were buried in graves, some with traces of severe abuse.

Hoffmann: Gather evidence and record witness statements

HRW regularly publishes reports on crimes against humanity in times of war. The organization condemned the use of banned land mines by both Russia and Ukraine. Unlike Ukraine, Russia has not signed the Convention on the Ban on Anti-Personnel Mines. Nevertheless, according to HRW, their use violates international law.

In the film, HRW experts Richard Weir and Sam Dubberley also commemorate the devastating use of cluster munitions at the Kramatorsk train station in the Donetsk region in April. Hundreds of people were trying to flee by train when a rocket hit and released dozens of small projectiles. "That's why 61 people died," says Dubberley.

Chief Public Prosecutor Hoffmann makes it clear that the main thing now is to collect evidence and record witness statements on video. Possible Ukrainian crimes would also have to be investigated. "The focus today is on documenting everything securely so that it can also be used in court in 20, 30 or 40 years." But it is also true that not everything can be clarified and will come before a court. "Of course you have to realize that individual soldiers who have been identified as perpetrators may have been dead for a long time."

Moscow raises counter-accusations

And Russia? The power apparatus in Moscow remains true to its line of dismissing allegations or blaming Ukraine for the crimes. Moscow has complained about the execution of Russian prisoners of war and the stationing of Ukrainian tanks and other military targets in residential areas and schools. Above all, Russia has repeatedly tried to justify the invasion by saying that blood has been flowing in eastern Ukraine since 2014, and that Kiev was already sending troops back then to defeat pro-Russian separatists in the Donbass region.

For eight years now, Moscow has been accusing Kiev of trying to use armed force to prevent the Russian-speaking population in Donbass from self-determination. The Russian Foreign Ministry is showing a collection of unverifiable images of destruction and blood on its website that are intended to prove Ukrainian crimes.

From Moscow's perspective, Kiev's crackdown on Russian and other national minority languages ​​was the spark that ignited the conflict. Minorities complain that forces that came to power in the 2014 ouster of pro-Moscow President Viktor Yanukovych were having their rights curtailed.

The Hague needs very clear case

But there is no justification for this war. From the point of view of experts, the number of Russian war crimes is so high that it is now primarily a question of clarifying individual concrete acts as completely as possible. The international law professor Claus Kreß from the University of Cologne says in the ARD film that a very clear case must be found for charges to be brought before the International Criminal Court in The Hague. That is difficult.

Russia does not recognize the jurisdiction of the court. In addition, the immunity of Putin and other Russian officials is considered an obstacle. This is another reason why a special tribunal is being discussed more and more intensively internationally, because the crime of aggressive war or aggression is considered easier to prove.

"You can say that the Russian war of aggression is the original sin, which then gave rise to the whole flood of further crimes," Kress sums up. "The crime of aggression is about holding accountable those state leaders who make the decision to go to war in flagrant violation of international law," said the international law expert. "It takes nothing away from the cruelty of every single war crime if you realize: Without the decision to go to war of aggression, the gates to all the many atrocities in war would not have been opened in the first place."