CAIRO —
American citizen Mustafa Kassem, who has been imprisoned in Egypt since 2013, has died in detention, his lawyers and a source close to the family said on Monday.

He died of heart failure following a hunger strike, according to Pretrial Rights International, which represents Kassem. The 54-year-old is believed to have started a hunger strike in September 2018.

“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),” Pretrial Rights International said in a statement.

“The Egyptian foreign ministry informed the family around three hours ago that Kassem has died,” a source close to the family, who requested anonymity, told ABC News.

“The authorities did not provide any reasons for his death, which is always the case in such incidents,” the source said.

David Schenker, Assistant Secretary of State for Near Eastern Affairs, said he and the U.S. State Department must continue to raise “our serious concerns” about the human rights of Americans detained in Egypt.

“I’m deeply saddened today to learn of the death of U.S. citizen Mustafa Kassem … His death in custody was needless, tragic and avoidable,” Schenker told ABC News.

A senior State Department official said it’s “still premature to talk about” U.S. punishment for Egypt for Kassem’s death. “We are really concerned about this, and we’re going to talk about it.”

On Aug. 14, 2013, the night before Kassem was set to return to the U.S., he went out in Cairo to exchange money and shop, when security officials detained him and accused him of participating in protests against a military takeover in a nearby square, according to Praveen Madhiraju, one of his lawyers.

One month earlier, the military ousted Islamist President Mohamed Morsi after mass protests against his rule.

Kassem’s brother-in-law, who accompanied him on a visit to Cairo, was also detained on the same day when Egyptian security forces crushed a pro-Morsi sit-in, killing more than 800 people according to human rights groups. He was later released.

Kassem, a New York City taxicab driver, started his hunger strike after being convicted of trying to overthrow the government of Egyptian President Abdel Fattah el-Sisi and sentenced to 15 years in jail in a mass trial with more than 700 co-defendants.

In February 2019, Kassem’s sister said he was dying because of his hunger strike.

When he decided to go on a hunger strike, Kassem pleaded with U.S. President Donald Trump and Vice President Mike Pence to help free him.

“Dear President Trump: As an American beginning the hunger strike that could leave my two young children without a father and make my wife a widow, I pray that you secure my freedom,” he wrote in letters sent to Trump and Pence.

Egypt is notorious for its treatment of political prisoners, with human rights groups often calling on the country to provide medical care to detainees and investigate their claims of torture. It is unknown how many U.S. citizens are detained in Egypt.

“A diabetic with a heart condition, prison officials limited access to necessary medications and medical care for the entirety of his (Kassem’s) detention. He remained in pretrial detention for over five years,” Pretrial Rights International’s statement read.

In June last year, ex-president Morsi died after collapsing during a court hearing. His family, who accused authorities of denying him proper medical access, said he had fainted twice and experienced a diabetic coma in 2017.

“The death of former President Morsi strikes some frightening parallels with Mustafa’s case. Mustafa is also being housed at Scorpion prison within Tora Prison Complex. He also has a heart condition and diabetes and is subject to similarly neglectful medical treatment,” Madhiraju said at the time.

ABC News’ Conor Finnegan contributed to this report.