Egypt said Tuesday it would investigate the death in custody of a US citizen who had gone on a hunger strike as part of a six-year battle against his detention. Mustafa Kassem, 54, an Egyptian-born auto parts dealer from Long Island, New York, died late Monday of heart failure after a hunger strike he began last year, his lawyers said. Egypt’s chief prosecutor ordered an autopsy and said officials are questioning all doctors who oversaw Kassems care in prison and at the Cairo University hospital where he died. In Washington, Assistant Secretary of State for Near Eastern Affairs David Schenker told reporters that Kassem’s death was “needless, tragic and avoidable.” Kassem, who suffered from diabetes, was in Cairo to visit family in August 2013 when his lawyers say he was mistakenly swept up in a vast dragnet during the dispersal of a Muslim Brotherhood sit-in.