In late April 2022, a Syrian defector leaked a video of the Tadamon massacre. Hundreds of Syrian families watched the clip, hoping to learn what had happened to their missing sons.

The Siyam family watched it, too. Their son, Waseem Siyam, left his home in Damascus in the early morning of April 14, 2013. He had been ordered by the government to deliver flour to a state-run bakery in the city's southern Tadamon neighborhood. The 34-year-old never returned from what should have been a routine task.

For many years, the Siyams, who now live in Germany, believed Waseem had been arrested at a checkpoint and taken to a government jail. Yet the leaked video finally revealed to them the grisly details of his disappearance.

The clip shows a blindfolded man in a white t-shirt and jeans being led through an empty alley to a pit filled with corpses.

"My father was the first to recognize him [in the video]," says Tasnim Siyam, Waseem's sister, adding that her father recognized his son by his gait. "He looked so different, they had beaten him already, maybe it was also the fear."

They made Waseem jump into the pit and shot him dead.

"It is surreal, how can I process the fact the man being killed in the video is my brother," asks Tasnim Siyam.

Toying with victims

The leaked video was recorded on April 16, 2013, just days after Waseem's disappearance. It shows two uniformed men methodically kill 41 people. Each time, one of the two gets a blindfolded person from a white delivery van, leads the person to a large pit that already contains several corpses and car tires. They are pushed in and shot dead. Then, the bodies are doused in fuel and set alight.

The Syrian war has now been raging for 11 years, during which countless atrocities have been committed by all parties involved in the fighting. The Tadamon massacre video, recorded in what was then government-controlled Damascus, however, stands out for the planned nature of the callous brutality it shows.

It is clear from the video that the two executioners are enjoying their "work." They murder civilians in broad daylight in a calm, routine manner. They play a twisted game with their victims, falsely telling them the alley they are passing through is being targeted by snipers, so the blindfolded victims run along and fall into the pit. Before they know what has happened, they are shot.

The murderers, feeling untouchable, documented their crimes by filming high-definition footage. Sometimes they wave at the camera and crack jokes. Why did they record their crimes? Did they want to produce "war trophies"? Or show their superiors they were doing as they were ordered? Whatever the reason, it's highly unlikely they thought the video would be seen by the public.

Identifying the killers

Ugur Umit Ungor, a professor of Holocaust and genocide studies at the University of Amsterdam, was among the first to receive a copy of the clip in 2019. Working with his colleague Annsar Shahhoud, the two were able to identify the two killers in the video: Najib al-Halabi, who is now dead, was part of a militia loyal to Syrian President Bashar Assad; the other man, seen wearing a fishing hat, is Amjad Youssef. He is an officer in Assad's secret service.

"The last two years were hellish for us," Ungor tells DW. "Imagine you know of a horrible massacre, need to watch this video again and again, but cannot tell anyone else about it."

Analyzing the video to identify the murderers was nothing like ordinary academic research. They set out to do what so far few have managed to achieve: Produce definitive proof that the Syrian state is directly responsible for some of the worst atrocities committed in the war.

'I killed many people'

Ungor and his Syrian colleague Shahhoud used unconventional research methods. Shahhoud adopted a fake online identity to dupe the Syrian regime and find out more about the men behind the massacre.

Shahhoud pretended to be a young Alawite woman from Homs by the name of "Anna Sh" who supported Alawite President Assad. The Guardian newspaper first covered how, over the course of two years, Shahhoud succeeded in talking to hundreds of Assad staffers and winning their trust. One day, she stumbled across Amjad's online profile. Shahhoud, as Anna, befriended the man. Occasionally, they would chat on the phone, with Ungor secretly listening.

Amjad Youssef told her he felt lonely and stressed. Shahhoud's assumed character lent him her ear. After several months, he confided in her: "I killed many people." The two academics had finally achieved their goal.

The two passed the recorded conversations to Dutch and German prosecutors, and described their findings in New Lines magazine.

"We could not lean back and say: We are academics, we do research, let this revelation blow up on social media and not get involved," says Ungor. "We had to take responsibility."

Solid evidence of war crimes

The leaked six-minute video presents solid evidence of crimes as they were committed by identifiable perpetrators and victims. It also shows the killings were a pre-mediated massacre. A mass grave had been prepared, both killers acted in a routine fashion and attempted to conceal their victims' identities.

The video suggests the killings followed a systematic pattern, said Alexander Schwarz, an expert on international criminal law with Amnesty International. "A systematic attack on civilians, as seen in this video, is one criterion for a crime against humanity."

Still, it is unclear whether these revelations will lead to a trial. A life sentence handed down to Syrian officer Anwar Raslan in January by a German court in Koblenz, however, shows the surviving executioner in the video could be prosecuted.

Additionally, the video is of great political significance, according to Fritz Streiff, who provides legal counsel to Mnemonic, an organization documenting human rights abuses. He argued that its shocking nature shows that nations should not normalize ties with the Assad regime.

"This is not just about Amjad Youssef," says Ungor. "This is also about the system that creates people like him."

Closure for families?

Schwarz and Streiff work with people whose relatives have vanished in Syria. Shedding light on the disappearance of loved ones, as the Anwar Raslan case did, can bring closure to some families.

Tasnim, the sister of murdered Waseem, is not so hopeful.

"I know you need to stay positive in life, but we have lost all hope," she says. "We're alive only because we want to be.

"I want people to understand why some of us need months, or years, to settle down [in Germany]," she says. "Some never settle down, because they endured things in Syria that were so horrific no normal person could ever imagine them."

This article was originally written in German.