Egyptian-American Moustafa Kassem has died after more than six years in custody in Egypt, according to officials.

Egypt's attorney general said in a statement that the dual citizenship holder died in a hospital in Cairo on Monday, a day after being transferred there from prison. An autopsy was ordered to determine the cause of death.

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According to Kassem's daughter, Dima, his health deteriorated after launching a hunger strike inside the prison to protest against his arrest during a 2013 crackdown by security forces against supporters of former President Mohamed Morsi after the army toppled him. Kassem had long insisted he was falsely charged and had no links to any opposition groups.

Speaking to Al Jazeera, Dima alleged that her father had suffered from a case of intentional medical negligence, which led to his death. She also said that Kassem was first imprisoned for a period of five years without being formally charged.

Kassem had reportedly been on a liquid-only hunger strike since being convicted in September 2018 and four days ago ceased drinking fluids.

The United States confirmed his death and said it would continue to raise concerns over Egypt's human rights record, which rights groups have repeatedly criticised over the government's treatment of political detainees, dissidents, journalists and human rights defenders.

"I am deeply saddened to learn today the death of US citizen Moustafa Kassem who'd been imprisoned in Egypt," the US assistant secretary for Near Eastern Affairs, David Schenker, told a State Department briefing on Monday.

"His death in custody was needless, tragic and avoidable," Schenker said. "I will continue to raise our serious concerns over human rights and Americans detained in Egypt at every opportunity."

Commenting on the news, US Senator Chris Murphy said in a Twitter post that Kassem should have "never been detained".

"[Mike] Pompeo must remind Egypt that military aid is legally tied to releasing prisoners, including at least 6 US citizens," Murphy said, referring to the US secretary of state.

Egypt has long enjoyed military and economic aid from other countries, including the US.

Sahar Aziz, a professor at Rutgers University, said Kassem's death was "just another tragic" one among political prisoners in Egypt.

"The fact that a person would rather starve to death than stay in an Egyptian prison, speaks volumes of just how horrendous the conditions are in those prisons,” Aziz told Al Jazeera.

"I think the fact that he was an Egyptian American also speaks to [US President] Trump's dehumanisation of Arabs and Muslims, and so one wonders if he had been of American or European decent, Trump and Bennett would be more aggressiv," she added.



'Find out why'

According to reports, Washington previously pressured Egypt into releasing Aya Hijazi, a dual US-Egyptian citizen, who was acquitted in 2017 after nearly three years of detention over accusations related to running a foundation dedicated to helping street children.

At the time of her arrest, local human rights groups said the charges were fabricated and part of a crackdown by the Egyptian government of President Abdel Fattah el-Sisi on civil society groups.

Hijazi wrote on Twitter late on Monday that Kassem had died "from a strike". She called on US President Donald Trump to "find out why" and "how he died", and to "ask about the others in Sisi's prisons".

Qassem is undergoing a second autopsy. Thanks to all who pressured. @realDonaldTrump couldn’t save his life, but I hope he works to find out why&how he died &ask about the others in Sisi’s prisons inclusion Reem & Ola ElQaradawi



شكرا لكل من ضغط لإعادة تشريح جثة مصطفى قاسم pic.twitter.com/8EcZFnKyPo — Aya Hijazi آية حجازي (@_AyaHijazi_) January 14, 2020

Kassem was sentenced to prison over the sit-in at Rabaa al-Adawiya Square in Cairo that ended with security forces killing hundreds of protesters - the single most deadly incident in unrest that followed the 2011 popular uprising that toppled Egypt's longtime leader Hosni Mubarak.

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.

In 2018, US-based rights group Human Rights Watch called for an international inquiry into the deadly crackdown, saying that Egyptian authorities had failed to investigate or prosecute a single member of the security forces responsible for the assault.

The bloody dispersal came weeks after Morsi, a member of the Muslim Brotherhood, was overthrown following mass protests against his one-year rule. Egypt's government said many demonstrators were armed and 43 police officers were killed.

Aside from the mass convictions, an Egyptian court in July 2018 sentenced 75 people to death for participating in the protest.

Rights activists say el-Sisi has overseen an unprecedented crackdown on freedoms in Egypt since first taking power in 2013 and then winning elections in 2014. El-Sisi and his backers say the measures are needed to keep Egypt stable and to counter threats from armed groups.