(CNN) A US citizen who once begged for President Donald Trump's help has died after more than six years in an Egyptian prison.

Mustafa Kassem died of heart failure on Monday, according to Pretrial Rights International and The Freedom Initiative, the two organizations that represented him during his case. He was 54 years old.

Kassem, who was Egyptian American, was detained in August 2013 in Cairo while visiting his family. He was beaten by security forces and held in pretrial detention for more than five years before being sentenced without due process in September 2018 to 15 years in prison, according to the organizations.

Shortly after his sentencing, Kassem sent a handwritten letter to Trump to inform him of his plight and implore the US President for his help.

"I pray that you have a plan for me," Kassem wrote. He told Trump that he was diabetic and was going on a hunger strike "knowing full well that I may not survive it."

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