Khartabil's fate had been unknown to relatives and fellow activists for nearly two years. According to a Facebook post on Tuesday from his wife, Noura Ghazi Safadi, he was taken to Syria's Adra prison -- one of the country's most notorious -- in October 2015, and executed days after.
"This is the end that suits a hero like him. Thank you for killing my lover. I was the bride of the revolution because of you. And because of you I became a widow," Safadi wrote on Facebook, in a post seemingly directed at the Syrian government.
CNN has not independently confirmed Khartabil's death.
"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. Our thoughts are with his family," Anna Neistat, Amnesty International's senior director of research, said in a statement Wednesday.
Khartabil 'opened up the internet in Syria'
Khartabil, also known as Bassel Safadi, was a Syrian-Palestinian who was widely seen as the person who brought open-source software to Syria, helping Syrians evade regime surveillance and online censorship.
Foreign Policy magazine ranked the activist as the 19th most influential thinker in 2012 for "insisting, against all odds, on a peaceful Syrian revolution."
Khartabil was arrested in Damascus in March 2012. A year after his arrest, two members of the European Parliament, Charles Tannock and Ana Gomes, credited him with "opening up the internet in Syria ... and vastly extending online access and knowledge to the Syrian people."
In a letter to the European Commission, the MEPs said Khartabil's arrest was "strongly suspected" to be "part of an effort to restrict access to online communities and discourses and stifle free expression in Syria." They also said Khartabil was believed to have been tortured in prison.
In an address on International Human Rights Day 2015, then-US Secretary of State John Kerry paid tribute to Khartabil as a "prisoner of conscience."
The MIT Media Lab offered Khartabil a research position
in October 2015 -- the same month Khartabil is believed to have been executed.
Khartabil: Great friend to 'open knowledge community'
"Bassel was Creative Commons' Syrian project lead, an open source software programmer, teacher, Wikipedia contributor, and free culture advocate. He was also a devoted son and husband, and a great friend to many people in the open knowledge community around the world," open-internet non-profit Creative Commons wrote in a tribute to Kharatbil on Tuesday.
"Creative Commons, and the global commons of art, history, and knowledge, are stronger because of Bassel's contributions, and our community is better because of his work and his friendship."
"His death is a terrible reminder of what many individuals and families risk in order to make a better society."
Tuesday's reports of Khartabil's execution bring to an end five years of social media campaigning around the hashtag #FreeBassel calling for his release.
Amnesty International estimates
that 17,723 people died in Syrian prisons between March 2011 and August 2016, an average rate of more than 300 each month.