TEL AVIV, Israel — Nineteen Israeli prison guards are under investigation in the death of a 38-year-old Palestinian security prisoner in their custody, police said Thursday.

Thaer Abu Assab was found with “severe signs of violence” on his body, according to the Palestinian Prisoners’ Club, an advocacy group. He died on Nov. 18 at the Ketziot prison in the southern Negev Desert. Abu Assab was arrested in 2005 and was serving a 25-years sentence for attempted murder.

The Prisoners’ Club demanded an investigation, including an autopsy. Police originally told the group that it was difficult to identify the guards involved due to their helmets.

Police announced the investigation of the guards Thursday, after a gag order expired. The Israel Prison Service said it was cooperating.

The Prisoners’ Club says about 7,800 Palestinians are currently held in Israeli prisons. Hundreds were rounded up since the Oct. 7 start of the Israel-Hamas war in Gaza, triggered by a deadly attack by Hamas militants in southern Israel.

Rights activists say the number of so-called administrative detainees, who are people held without trial or charges, is a record high.