In a drab office building in a European capital amid the sound of humming document scanners, a team of human rights lawyers is hard at work processing thousands of documents that they say link the Syrian government to war crimes.
The papers point to an unmistakable conclusion, according to those leading the effort: The government of Syria's President Bashar al-Assad has violated the international rules of war through attacks on civilians, torture, rape, and the use of chemical weapons, among other crimes.
"We have stronger evidence than we had for any past conflicts, any past tribunals, any past international justice efforts," said Chris Engels, deputy director of the Commission for International Justice and Accountability, which has been documenting human rights abuses by Syrian officials since the start of the civil war in 2011.
According to CIJA adviser Stephen Rapp, the Syrian government meticulously documented its treatment of thousands of detainees — a product of its large bureaucracy. As a result, thousands of leaked photos mean prosecutors have far stronger evidence of war crimes than what existed to convict the Nazis at Nuremberg, said Rapp, the former U.S. ambassador-at-large for war crimes issues under President Barack Obama.
The documents amassed so far form a "paper trail of war crimes" noteworthy for their specificity, according to Engels, who requested NBC News not reveal the location of the group's office out of concern for investigators' safety and the security of the evidence. The documents are stored in a vault.
Click to see the graphic, smuggled images:
The group's 140-person staff is made up of lawyers, investigators, and translators. That includes more than 40 "document hunters" in Syria whose mission is to extract material produced by the regime, authorizing the detention, torture, and execution of people for suspected anti-government activity.
More on MSNBC's "On Assignment with Richard Engel" Friday at 9 p.m. ET
Documents identify who in the regime signed off on what, who was targeted, and why. In some cases where the documents indicate that a prisoner admitted to participating in anti-regime activity, CIJA tracked down the accused, who later said their confessions were extracted through torture.
The organization was founded by Bill Wiley, a Canadian former war crimes investigator, as news reports began surfacing of widespread abuses by the Assad regime in 2011. It receives funding from Western countries, including the British and Canadian governments, to carry out its work.
Added to the trove of documents is vital visual evidence: roughly 50,000 photos shot between the start of the war in 2011 and 2013, cataloging more than 6,700 victims of torture by pro-regime forces. They were taken by a forensic photographer known by the pseudonym Caesar who worked for the Syrian military and smuggled them out of the country on hard drives in 2013.
Assad has disputed the veracity of the photos, but Human Rights Watch called the images authentic. The New York-based group confirmed the identity of 27 victims in the photos through interview with relatives, while former prisoners and defectors have corroborated the widespread torture in government prisons. In a separate analysis, the FBI confirmed a portion of the photo trove as authentic. So did a U.N. report.
Caesar's photos show emaciated corpses, many stripped naked, arrayed on the ground — some with gouged-out eyes, bloodied genitals, and severed fingers. Many show signs of bruises, burn marks, and gashes covering discolored skin. While most of the images are too graphic to air or publish, NBC News has included two below to illustrate the treatment thought to have been meted out to Syrian detainees.
Other investigators and activists have also collected testimony, images and videos documenting atrocities committed by all sides during Syria's war, a U.N. quasi-prosecutorial body has said. The team announced Monday that it is preparing case files and has engaged with war crimes investigative units of different countries including in Europe, whose courts can exercise universal jurisdiction to prosecute.
CIJA's goal is to prove government officials' individual criminal culpability, from the highest echelons in Damascus down to the provincial level. The Syrian government's formalized command-and-control structure, as well as its careful record-keeping, make that work easier.
The Syrian government is "process driven," Engels said. Directives — even those authorizing torture — were documented "to make sure that everyone does what they're supposed to be doing."
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CIJA's work is logistically difficult and comes with significant risks. Over the course of the war, where rebels seized regime territory, CIJA's document hunters entered abandoned government facilities and extracted material — USBs, computer hard drives, reams of paper — that might contain evidence of war crimes.
Next they had to safely store the material, and when possible, smuggle it through checkpoints and out of Syria. Engels says no one directly employed by CIJA was ever killed in the process, but people who worked closely with the team were.
Once the documents are ferried out of Syria, the rest of the CIJA team sifts through them to build cases for future criminal prosecutions.
Despite what CIJA and other investigators consider to be clear and overwhelming evidence of war crimes by Assad's government, the regime remains more firmly entrenched than at any other point in the war due in large part to the support of Russia, Iran and Iran-backed Lebanese militia Hezbollah.
Now a concern among human rights lawyers pursuing a case against Assad's government is that victims of the regime will never see officials held to account.
Criminal prosecutions within an international tribunal seem increasingly unlikely. For a case to come before the International Criminal Court, it has to be referred there by the United Nations Security Council. Russia and China, two of five veto-wielding members on the council, quashed such a move in 2014.
"They are dead-set against justice," Rapp said. "They are dead set against any kind of investigation that's independent."
So in place of an international tribunal, Engels and Rapp have been focusing on lower level domestic prosecutions, and are helping authorities issue arrest warrants and build cases against Syrian government functionaries who have since left the country for the West. Last year alone, Rapp says he fielded 400 requests from law enforcement officials in 12 different countries, including the FBI.
In September, prosecutors in Sweden issued the first war crimes conviction of a Syrian army soldier who had sought asylum in the country: eight months in prison for violating personal dignity by posing with his foot on five corpses in a photo.
Rapp and Engels admit that such small fare is an unsatisfactory answer to the millions displaced, wounded or left grieving loved ones killed in the war and who want to see Assad held accountable.
"It won't be as good as in an international court, but that's the only alternative we have right now," Rapp said.