An American imprisoned in Egypt for more than six years on what he insisted were false charges, died on Monday after a long hunger strike, the State Department has said.
Moustafa Kassem, 54, a dual Egyptian-American citizen, was arrested in Cairo in August 2013 following a military coup that brought Abdel Fattah el-Sisi to power.
Kassem had insisted he had no links to opposition politics and had been wrongfully detained by Egyptian soldiers when he happened to be at a shopping centre near Cairo's Rabaa al-Adawiya square.
The dispersal of a sit-in at Cairo's Rabaa al-Adawiya square on 14 August, 2013, saw soldiers and police shoot dead more than 800 protesters in a matter of hours, and arrest thousands.
The bloody crackdown came weeks after the ouster of Egypt's first democratically elected President, Mohamed Morsi.
Human Rights Watch concluded that the killings "likely amounted to crimes against humanity" and "were part of a policy to attack unarmed persons on political grounds."
After spending more than five years in pretrial detention, where he said his diabetes and a heart ailment went largely untreated, Kassem was convicted and sentenced to 15 years in jail in a mass trial involving hundreds of defendants.
Soon after that, Kassem went on the first of several hunger strikes, refusing solid food for months on end to protest what he called his unjust imprisonment.
He wrote letters to both US President Donald Trump and Vice President Mike Pence, asking them to secure his freedom and not make his wife a "widow".
"I am deeply saddened to learn today the death of US citizen Moustafa Kassem who'd been imprisoned in Egypt," Assistant Secretary for Near Eastern Affairs David Schenker told a State Department briefing on Monday.
"His death in custody was needless, tragic and avoidable," he added.
Kassem's brother-in-law Mustafa Ahmed, had previously described conditions at the maximum security Tora prison, where he was kept, as dire.
"The cells are filthy, infested with insects, rodents and snakes," he wrote in an op-ed for The New York Times last year.
"They have no ventilation, sun or light. Kassem and the other prisoners have no access to clean water, a bed, a chair or any books."
Since 2013, Sisi's government has overseen a broad crackdown against dissent, with as many as 60,000 people jailed.
While members of the Muslim Brotherhood - to which President Morsi belonged - were the main target, secular and left-wing activists have also been imprisoned.
Morsi died in jail last June, after enduring almost six years in solitary confinement.
A 2018 report by members of the UK Parliament had warned that the "cruel, inhuman and degrading" conditions of Morsi's detention, including lack of medical care, may lead to his death.
Sisi won a second term in March 2018, in what critics called a "sham" election. He secured more than 97 percent of the vote.
The presidential elections featured only one other candidate - Moussa Mustafa Moussa - an ardent Sisi supporter who once formed a campaign group called: "Supporters of President Sisi's nomination for a second term."
Egypt is the second-biggest recipient of US military aid after Israel, receiving $1.3 billion every year.
Trump has repeatedly heaped praise on the Egyptian leader, even calling him "my favourite dictator".
