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.

Documents being smuggled out of Syria by the Commission for International Justice and Accountability. Commission for International Justice and Accountability

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.

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.

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.