Mazen Darwish cowered in the corner of a blood-stained cell in the Syrian capital Damascus.

"Tomorrow, I'll be free," he told himself.

Deep down, he knew this was not true, but holding onto the faint glimmer of hope kept him alive. He had not showered in months and felt weak from food deprivation. Every day, he was beaten with clubs, shocked with electric prods and hung by his arms from the wall, Darwish recalled.

"The guards used torture for torture's sake - not to get information, but to humiliate and destroy us," he told Al Jazeera.

The prominent human rights activist spent three and a half years in government custody. In August 2015, he was released.

"Regime members have acted with impunity for years now," said Darwish, 43. "Any political solution to the Syrian conflict without accountability and justice won't bring sustainable peace."

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Earlier this year, Darwish and eight other Syrian former detainees submitted a criminal complaint against six high-ranking military intelligence officials close to President Bashar al-Assad, accusing them of war crimes and crimes against humanity.

For four months, Germany's federal prosecutor has been hearing witness testimony from the survivors, most of whom now live as refugees in the country. The case is ongoing.

The Berlin-based European Center for Constitutional and Human Rights, which has been supporting the Syrian group's complaint, sees this as a first step on the road to justice. The centre's lawyer, Patrick Kroker, hopes the case will lead to international arrest warrants.

"Torture has been used in Syria for decades, but in March 2011 it was implemented as a political response to anti-government protests," Kroker told Al Jazeera.

"Human rights organisations have raised awareness [of] torture, chemical attacks, air strikes targeting hospitals and the siege of Aleppo. No one can pretend to not know what is going on inside Syria. We must act now. Legal boundaries have to be upheld, even in times of war," he added. "War cannot be a black hole."

In their testimony, the former detainees have described instances of brutal beatings, sleep deprivation, having their skin chemically burned with detergents, sexual violence, and being held in inhumane conditions in overcrowded, rat-infested cells in three Damascus jails between October 2011 and July 2015.

Their case is unique because it was launched under the principle of pure universal jurisdiction, which only a few countries in the world have implemented, allowing prosecutors and courts to investigate international crimes even in the absence of links to Germany. The International Criminal Court at The Hague cannot investigate or prosecute crimes committed in Syria because the country's government has never acknowledged the court's jurisdiction. While the United Nations Security Council could refer the case to the ICC, Russia, an Assad ally, blocked such a move with its council veto.

Darwish, a lawyer and pro-democracy activist, openly criticised the Syrian regime's crackdown on anti-government protests in March 2011. In February 2012, security forces stormed the Damascus office of his organisation, the Syrian Center for Media and Freedom of Expression. Darwish, his wife and other members of the centre were arrested.

In the ensuing years, he was transferred from one detention centre to the next, including a military branch, an air force security camp and a state security branch. Within the first year, Darwish said, he lost 90 pounds and a skin infection spread across his body. "I looked like a skeleton," he said.

Hundreds of thousands of civilians have been caught in the crossfire of the Syrian war, now in its seventh year. According to the Syrian Observatory for Human Rights, more than 465,000 people have lost their lives in the deadliest conflict of the 21st century, while 12 million Syrians have been displaced from their homes.

Rights groups have compiled evidence of systematic torture and extrajudicial killings, including mass hangings in a network of government-held detention facilities, such as the notorious military prison Sednaya. Since 2011, more than 17,000 people have died in government custody, according to Amnesty International - but it is impossible to determine how many people have been jailed overall.

"This case is a sign that the wall of impunity around Syria is cracking," Sara Kayyali, a Syria researcher at Human Rights Watch, told Al Jazeera. "It's one of the first times that high-level Syrian regime members are being investigated for war crimes of torture."

The investigation in Germany shows that Europe and the international community is taking the alleged crimes committed in Syria seriously, she added.

"Realistically, you can't deny that it's a long road to justice. More has to be done, certainly, but it's a step in the right direction," Kayyali said. "The human rights abuses and war crimes in Syria aren't a secret that the world community can turn a blind eye towards."

In 2014, a former Syrian military police photographer released more than 50,000 photos under the pseudonym Caesar, showing the mutilated bodies of thousands of men, women and children who died in detention. The Syrian regime has repeatedly denied such allegations.

EXPLAINER: Syria's civil war

Khaled Rawas, a 28-year-old complainant in the case in Germany, says he does not seek revenge: "It's about justice."

Rawas, an engineering student, was detained twice because of his political activities in Syria. In December 2011, he was arrested during an anti-government protest in Damascus and jailed for 28 days, during which time he was beaten with a pipe, subjected to electric shocks and forced to watch other prisoners getting hit with spiked sticks.

"Their screams sent shudders down my spine," he told Al Jazeera, his voice cracking.

"I will never forget that sound ... Many Syrians I know don't understand why I've joined the case, because they don't believe it will make a difference. Too many people have suffered for too long, while the world hasn't intervened. Thousands are still being tortured. We have to give people hope again. This case is about saving humanity."