A Palestinian-Syrian software pioneer has been executed in prison after being arrested five years ago by Syrian authorities in Damascus, his widow and colleagues said on Wednesday, in what Amnesty International said was a “grim reminder of Syrian prison horrors.”

Noura Ghazi Safadi wrote on Facebook late Tuesday that she has received confirmation that security services executed Bassel Khartabil in October 2015 after torturing him in prison.

Khartabil, who also went by the name Bassel Safadi, was a champion and leading contributor to Arabic Creative Commons, a framework for coding and legal rights that promotes the open distribution of software and ideas, according to his Lebanese friend Dana Trometer.

Khartabil was taken from the street in Damascus in March 2012 amid a wave of military arrests, Creative Commons said in a statement on Wednesday confirming news of his execution. He was jailed for several years, during which time he was allowed to infrequently communicate with family members. Then, in October 2015, he was abruptly transferred to an undisclosed location and all communications with the outside world ceased, it said.

Oussama Jarrousse, a Berlin-based colleague of Khartabil, said he was not only a highly skilled software developer but he was also knowledgeable in the policy side of the internet, and was well integrated with the international “Open Internet” community, like Wikipedia, Mozilla, Creative Commons, and had a good reputation among his international peers.

“It takes years of hard work and effort to build such trust and relationships,” he said.

“We are deeply saddened and outraged at this awful news. Bassel Khartabil will always be remembered as a symbol of courage, who peacefully fought for freedom to the very end,” said Anna Neistat, Amnesty International’s senior director of research.

In a statement, she said “his death is a grim reminder of the horrors that take place in Syrian prisons every day.”

“The tens of thousands of people currently locked away inside Syrian government detention facilities face torture, ill—treatment and extra—judicial executions. These cruel acts undoubtedly amount to war crimes and crimes against humanity,” she said.

The London—based watchdog said in a report earlier this year that Syrian authorities have killed at least 13,000 people since the start of the 2011 uprising in mass hangings at a prison north of Damascus known to detainees as “the slaughterhouse.”

The February report covered the period from 2011 to 2015, when Amnesty said 20-50 people were hanged each week at Saydnaya Prison in killings authorized by senior Syrian officials, including deputies of President Bashar Assad, and carried out by military police.

The Syrian government rejected the Amnesty report and denied its findings.