The 45-year-old went on hunger strikes several times after previous arrests, including a 55-day strike in 2015 to protest his detention without charge.
A medic from the group Physicians for Human Rights Israel visited Adnan in prison this week and warned that he “faces imminent death” and called for him to be “urgently transferred to a hospital”.
The armed group Islamic Jihad warned Israel that it will “pay the price for this crime” as it called the father of nine a “martyr”.The Palestine ministry of foreign affairs has called for an international investigation into Adnan’s death, calling it a heinous crime.
“The Ministry of Foreign Affairs and Expatriates condemns in the strongest terms the heinous crime committed by the occupation authorities, which led to the martyrdom of Sheikh Khader Adnan,” the ministry said.
The Israeli military said missiles fired from the Hamas-controlled Gaza Strip fell in open territory but caused no damage.
A leader from the Fatah party in the occupied West Bank city of Jenin Jamal Huwail has told Al Jazeera Adnan represented an “icon of struggle and resistance for the coming generations”.
“This criminal Zionist occupation cannot be resisted except with perseverance, patience, resistance and confrontation,” Huwail added.
“I think what happened to Khader Adnan is an implementation of the current Israeli government’s policies – especially Itamar Ben-Gvir who is responsible for the prisons, and who has made matters more difficult for our prisoners. The occupation tried to send a message by assassinating Khader Adnan – a message of deterrence – to our heroic prisoners.”
Huwail said in the upcoming days “will be filled with a response to this occupation and the way it is treating our prisoners”.
According to reports, Adnan’s body was still being withheld by Israel authorities with no announcement of when it will be released.
The secretary general of the Executive Committee of the Palestine Liberation Organization (PLO) has said he held Israeli authorities “fully responsible” for Adnan’s death, blaming it on “negligence and forced detention”.
“My deepest condolences to his family, and we ask God to bless him with the martyrs and friends,” Hussein al Sheikh posted on Twitter.
Mustafa Barghouti, former Palestinian information minister and general secretary of the Palestinian National Initiative political party, says Adnan’s death “is a very dangerous thing that has happened”.
The Israeli government and its national security minister, Itamar Ben-Gvir, “are personally responsible for this act of assassination”, Barghouti told Al Jazeera.
“I call it an act of assassination because the Israeli government knew very well, and its military courts, that a person who is on hunger strike for 87 days, who had not received any kind of medical care, could die at any moment. And that’s exactly what has happened,” Barghouti said.
According to Israeli sources several settlers had been attacked in the northeast of the West Bank.
“Israel announced that three cars belonging to settlers were shot at, with one being lightly wounded,” he said, adding the attacker had fled the scene.