An international push is underway to hold Russian President Vladimir Putin and his most senior allies criminally responsible for the war in Ukraine.
Kyiv is calling for a special tribunal to prosecute them for the crime of aggression, considered the supreme international crime from which other war crimes follow.
A tribunal like this has not been held since senior German and Japanese leaders were tried in Nuremberg and Tokyo in the aftermath of World War II.
Governments, diplomats and legal experts are debating how to get the tribunal started while the world's attention is still focused on Ukraine.
One voice calling for a special tribunal is David Scheffer.
He served as US ambassador-at-large for war crimes issues under president Bill Clinton and has negotiated the creation of five war crimes tribunals for the former Yugoslavia, Rwanda, Sierra Leone and Cambodia, as well as the International Criminal Court (ICC).
"The ICC does not have jurisdiction over the crime of aggression with respect to the Russian invasion of Ukraine," he said, partly because Russia is not a member of the court.
"The crime of aggression is so blatant, it is so obvious, it is so self-evident with respect to Ukraine and the Russian invasion of it, that it would just be untenable that the crime of aggression would not be seriously investigated and prosecuted by some tribunal, but you have to build it for this circumstance."
Mr Scheffer and others believe the UN General Assembly can drive the process if a majority of its 193 member states recommend a tribunal be formed.
"The General Assembly has the authority to say, 'Yes, this will be a tribunal that will deal with the crime of aggression, investigate and prosecute it,'" he said, adding that it was to be expected that some states, such as Russia, would not cooperate.
"But that would not prevent, ultimately, indictments being handed down by the special tribunal."
Mr Scheffer wants to see the tribunal formally created by June this year, although it would take some time after that to be fully operational.
He acknowledges securing the required votes at the UN General Assembly is a highly political and laborious process, but he is urging the international community to act as soon as possible.
"You do not want to let this delay be too prolonged because remember, you want to be able to preserve evidence," he said.
"This will be a witness-driven investigation as well as [via] documents that may or may not still exist a year from now.
"So the sooner the better to actually formally bring the evidence together, including the interviewing of witnesses, and also while the world's attention is still focused on this."
Indictment would 'hang over [Putin's] head forever'
Veteran war crimes prosecutor Reed Brody knows justice can take years, or even decades, and patience and persistence are crucial.
He has been involved in cases against Chile's former dictator Augusto Pinochet, Jean-Claude "Baby Doc" Duvalier in Haiti and now the former president of Gambia, Yahya Jammeh.
Perhaps the greatest fight of his career was the 18 years he spent working to convict former Chadian dictator Hissène Habré of crimes against humanity.
"These [things] take time, however, and for those looking to prosecute someone today, we're likely to get the smaller fish," he said.
"We have already seen the prosecution of lower-level people. The Ukrainians are capturing Russian soldiers and mid-level people even, all the time.
"But the question of getting Vladimir Putin, the people around Vladimir Putin, if it's going to happen, it's likely to take a lot of time."
Even if Mr Putin is indicted for the crime of aggression, Mr Scheffer and Mr Brody point out he will likely remain out of reach in Russia. But they agree there is still value in pursuing him.
"Unless there's a change of regime in Russia, we won't see Putin in the dock of an international tribunal any time soon," Mr Brody said.
"But even if Putin is not arrested today, an indictment against him for war crimes or for aggression — crimes which have no statute of limitations — will hang over his head forever."
Moscow denies accusations of war crimes and continues to call its actions in Ukraine a "special military operation".
Mr Brody said while he supported the idea of a special tribunal in this case, he wanted to see other future cases of aggression prosecuted as well.
"There's already this perception, particularly in the Global South (regions broadly outside Europe and North America), that all these wonderful tools of international justice only kick in against enemies of the West," he said.
"You know, why is there this massive response, which I applaud, which we all applaud for Ukraine, but not for Yemen, not for Palestine, not for Ethiopia?
"If you're going to prosecute this act of aggression, there needs to be a commitment to prosecute all acts of aggression."
Documenting executions, torture and disappearances
The UN's Human Rights Monitoring Mission in Ukraine is one organisation gathering evidence of grave human rights violations committed during the war.
It has recorded the deaths of more than 7,000 civilians so far, including more than 400 children.
"We are documenting wilful killings, summary executions of both civilians and of prisoners of war," head of mission Matilda Bogner said.
"We are documenting enforced disappearances, arbitrary detention, we are documenting things such as torture and ill-treatment, again of both civilians and of prisoners of war."
The team monitors the conduct of both sides of the conflict and the information gathered often feeds back into criminal accountability efforts.
"We collect that information, we triangulate that information to ensure that it is credible, and then we report on it publicly," Ms Bogner said, "so that the world and the people in Ukraine understand what is happening here."
