The United States on Monday confirmed the death of Egyptian-American Mustafa Kassem in a prison in Egypt where he has been in custody since 2013, and vowed to continue raising Washington's concerns over Cairo's poor human rights record, Reuters reports.
"I am deeply saddened to learn today the death of US citizen Mustafa Kassem who'd been imprisoned in Egypt," Assistant Secretary for Near Eastern Affairs David Schenker told a State Department briefing.
"His death in custody was needless, tragic and avoidable," he said. "I will continue to raise our serious concerns over human rights and Americans detained in Egypt at every opportunity," Schenker said.
Kassem was sentenced to prison in September 2018, along with dozens of others over a 2013 sit-in which ended with security forces killing hundreds of protesters.
The sentencing, which included jail terms for more than 600 others, concluded a mass trial of people accused of murder and inciting violence during the pro-Muslim Brotherhood protest at Rabaa Adawiya square in Cairo.
Washington is Cairo's closest Western ally and one of its top aid donors.
Rights activists say Egyptian President Abdel Fattah al-Sisi has overseen an unprecedented crackdown on freedoms in Egypt since he took power in 2014. Sisi and his backers say the measures are needed to keep Egypt stable and to counter threats from Islamist militants.
Rabaa square was the single most deadly incident in unrest that followed the 2011 popular uprising that toppled Egypt's longtime leader Hosni Mubarak.