Egyptian-American citizen Mustafa Kassem has died in Egypt after going on hunger strike in prison.
The 54-year-old was a New York taxi driver, originally from Egypt, who was arrested on a home visit in 2013 on accusations of being a spy and taking part in anti-government protests.
Mr Kassem had always denied the charges against him.
He stopped eating solids late last year and ceased drinking fluids four days ago.
The top US diplomat for the Middle East, David Schenker, described Mr Kassem's death as "needless, tragic and avoidable".
Why was he arrested?
Mr Kassem was arrested in August 2013 in Cairo by security officials who accused him of being a spy and taking part in protests against the government of Egyptian President Abdel Fattah al-Sisi.
He was convicted and sentenced to 15 years in prison in 2018.
Mr Kassem - who was visiting family in Egypt at the time - denied accusations he was part of the protest, which left more than 800 people dead when security forces opened fire on demonstrators, according Human Rights Watch.
He accused Egyptian soldiers of snatching his American passport and stamping on it during his arrest.
How did he die?
Mr Kassem went on several hunger strikes during his detention.
Pretrial Rights International, which represented him, said he had died of a heart attack.
"Last Thursday, he ceased taking liquids and was shortly thereafter transferred to a local hospital, where he passed away today in the late afternoon (local time)," it said in a statement.
The Egyptian interior ministry said he was moved to the prison medical wing for treatment for his diabetes.
"His condition deteriorated... and he died on January 13," it said in a statement.
What has the reaction been?
Mr Schenker, the top US diplomat for the Middle East, called Kassem's death tragic and avoidable, but stopped short of outlining any repercussions for Egypt - a close US ally.
توفي الان المواطن المصري الأمريكي مصطفى قاسم والمحكوم عليه في قضية فض رابعة نتيجة الإضراب عن الطعام. سجين آخر تقتله الزنازين.— Aya Hijazi آية حجازي (@_AyaHijazi_) January 13, 2020
Mostafa Qassem, an American Egyptian, is yet another prisoner to die in Sis's prison. He died from a strike. #dead_in_prison pic.twitter.com/rkCTjhsYeP
Egypt's top prosecutor has ordered a post-mortem examination to determine cause of death, the official Egyptian news agency MENA reports.
"Another detainee killed by the prison cells," tweeted Egyptian-American activist Aya Hijazi, who was detained for almost three years in Egypt.