Israel has extended the detention without trial of Palestinian circus performer Mohammad Abu Sakha for another six months, his circus school said Monday.

Abu Sakha, 24, was arrested on December 14 in the West Bank, and held since January in administrative detention, the controversial measure under which Israel can hold suspects without trial for periods of six months, renewable indefinitely.

“Administrative detention order extended (for) another six months. Our hearts are heavy,” a statement from the Palestinian Circus School said.

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The Israeli army did not immediately confirm or deny the extension.

Amnesty International has called for Abu Sakha’s release, saying Israel believes he is a member of the Popular Front for the Liberation of Palestine, which is considered a terrorist group by Israel due to its armed wing.

Israel has not publicly confirmed the reason for detaining him.

The Palestinian Performing Arts Network, representing artists and cultural institutions, earlier this year said it was “deeply concerned” over the detention of Abu Sakha, who has been part of the Palestinian Circus School in Bir Zeit in the West Bank since 2008, first as a student and later as a clown and teacher.

Administrative detention is intended to allow authorities to hold suspects while continuing to gather evidence, with the aim of preventing further attacks in the meantime.

The system has been criticized by Palestinians, human rights groups and members of the international community.

More than 7,500 Palestinians are currently in Israeli jails, of whom around 700 are held under administrative detention, according to Palestinian rights groups. Another small number of Jewish Israelis are being held as well.