(CNN)Members of Syrian President Bashar al-Assad's regime could be prosecuted in national courts outside Syria -- and Russia could be vulnerable to prosecution for backing them -- a long-time American war crimes official told CNN's Christiane Amanpour on Tuesday.
War crimes ambassador: Russia, al-Assad regime vulnerable to prosecution
"These are men, women, and children tortured to death, eyes gouged out, etc. That is not the way that you build the future of Syria," said Stephen Rapp, who until August had served for six years as the U.S. State Department's ambassador-at-large for war crimes issues.
"This kind of criminal conduct can't be condoned, and people that go in on his side are endangering themselves in terms of being prosecuted."
"We've had these sorts of cases where people who support proxy forces can be held responsible. I don't think that should be the focus here, because the focus is al-Assad ... but it is important to recognize that going in to support him is also something in which you" allow human rights abuses to continue.
The focus is al-Assad, Rapp said, because he is responsible for the vast majority of abuses in Syria -- even as ISIS runs rampant over a huge territory of the country and grabs the world's attention.
Al-Assad has denied his regime uses the forms of warfare that have killed so many civilians -- like indiscriminate barrel bombing. Russia, for its part, says it is backing al-Assad only in his fight against "terrorists," despite international protestations that they have targeted the U.S.-backed opposition as well.
Last year, CNN's "Amanpour" program and the Guardian broke the story of a systematic torture program by the al-Assad regime, and the 55,000 gruesome photos taken by a Syrian government photographer-turned-defector -- codenamed "Caesar" -- that will almost certainly serve as evidence if there's ever a war crimes trial.
Rapp called those photos "the best evidence that I've seen in my career as a war crime prosecutor and ambassador."
A member of the legal team that investigated the torture photos, Geoffrey Nice, told Amanpour in January 2014 that all signs pointed to the al-Assad regime being responsible.
"If you have 11,000 bodies," Nice said, "dealt with in a systematic way -- brought from one place to another, where they were photographed with identifying marks ... to allow the authorities to give spurious explanations for the deaths of the people and to satisfy the authorities that people have been executed -- then you can reasonably infer that this is a pattern of behavior which has to have higher authority."
In January of this year, al-Assad told Foreign Affairs that the photographs were all "allegations without evidence."
Rapp is confident "the day will arrive" when there will be a criminal tribunal for al-Assad and those in the highest levels of the regime, as was done for the former Yugoslavia -- though he admits that's more of a long term proposition.
But in the short term, he said, there is a very real possibility that Syrians could be prosecuted in national courts, as opposed to international tribunals.
"Since 1984, all the countries in the world have recognized this as the kind of crime that can be prosecuted anywhere, regardless of whether the victim is one of your nationals or in your country."
In various countries -- including the U.S. -- "there are war crimes units that are prosecuting atrocity crimes that are committed elsewhere."
"Usually this is because one of the perpetrators comes to your county and he says he's a victim, but it turns out he's one of the butchers. So you prosecute him in country."
But, he said, "countries also have the ability to prosecute torture anywhere in the world if their country has an interest -- I mean a real vital interest in terms of its national wellbeing -- to taking that case."
With hundreds of thousands of refugees flowing into Europe, the case has never been stronger for those countries to bring cases.
"When you've got Germany talking about taking 800,000 refugees -- the reason those refugees are flowing is because of the torture, it's because of the barrel bombing, it's because life has become intolerable for people in that country."
They may not necessarily be able to go after Syria's top leadership, but certainly the people who are more directly responsible for the crimes depicted in Caesar's photos.
"On the Caesar photos, you see the unit where the person was killed; you know the date that it was taken; you know who was in command of that facility. Prosecute those individuals, put out their wanted posters, put out red notices under Interpol."
"Now, you won't have them tomorrow -- it took 15 years for the International Court for Yugoslavia to get [Bosnian-Serb General Ratko] Mladic -- but ... you basically send a signal that people that are involved in this kind of thing are going to be wanted for the rest of their lives."
"It's not as satisfying as having an international court and taking al-Assad or the leader of ISIS or others to justice, but it's something you can do today," Rapp said of prosecuting lower-level officials.
Rapp rejected the idea, increasingly floated by Western leaders, that it may be acceptable for al-Assad to stay on in Syria temporarily while the world deals with ISIS.
"Even a relatively brief future for al-Assad in country" is untenable, Rapp said.
"I talked to a defecting Syrian general the other day, he said: 'We dropped thousands of bombs on the opposition; we've dropped one on ISIS.' You see where the Russians are dropping the bombs."
The vast majority of Russia's airstrikes have been against the anti-Assad opposition, not ISIS, the U.S. and NATO have said.
"This guy's useless against ISIS. He's helped create ISIS. This is not the future of Syria. The future of Syria includes Alawites, includes people from his community and others, but that has to come together."
"It can't be the future. Getting there, I know, is hard. But getting there with him is impossible."
Rapp was one of the strongest voices in the Obama administration for keeping the al-Assad regime's crimes on the agenda. Nonetheless, the White House has come under severe criticism -- especially in the wake of Russia's intervention on al-Assad's behalf -- for not doing enough to stem the bloodshed.
Rapp, clearly emotional, tried to steer clear of the politics.
"When a country like us intervenes militarily, it has consequences. We become the target. It causes other kinds of dynamics. It's a difficult situation, I understand, from a military standpoint. I've stayed away from that."
"Frankly, I'd be sympathetic to ideas like no fire zones and no fly zones, and that kind of thing, but frankly the most important thing, from my point of view, is that we continue to talk the truth about what's happening there. And to recognize the suffering of the people of that country."
"It's not just a war, it's not just dealing with the counterinsurgency; it's a government that's gone all-in against its own people. And the response to that needs to be that has consequences, and we need now to do everything we can to ensure that there's a Syria in which there's justice."