The Joint List party on Monday sent a letter to the attorney general, urging that he open an immediate investigation into the circumstances surrounding the hospitalization of a terror suspect in critical condition after he was interrogated.

In the letter, the predominantly Arab faction said that Samer Arbid, the alleged ringleader of the cell accused of carrying out the August terror bombing, which killed 17-year-old Israeli Rina Shnerb and seriously injured her father and brother, had been in good health at the time of his arrest.

Arbid was hospitalized Saturday after an interrogation by the Shin Bet security service.

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The Joint List demanded that Attorney General Avichai Mandelblit review reports that the Shin Bet was authorized by the Justice Ministry to use illegal torture methods in its interrogation of Arbid.

The Justice Ministry has already launched an investigation into the circumstances of the 44-year-old terror suspect’s injuries, specifically probing the degree of force along with the tactics used by the Shin Bet interrogators.

“The conduct of the defense system, including the Shin Bet, in this case involved crossing a red line and breaking the law,” the Joint List’s letter stated, according to Hebrew-language media. “There must be a thorough examination of the circumstances in this case, and a conclusion must be reached. Measures must be taken to limit torture and to prosecute those involved.”

“Torture and the cruel and inhumane treatment of prisoners is in direct violation of international law and principles of human rights,” the letter continued. “We ask you to immediately intervene and direct all the professional bodies in your office to initiate an immediate investigation against those involved in the arrest, and the use of unreasonable force and torture.”

Hamas leader Ismail Haniyeh on Monday warned Israel it bears responsibility for the life of Arbid.

“The Zionist occupation will pay the price for its crimes against the prisoners,” the head of the Gaza-ruling terror group was quoted as saying by the Kan public broadcaster.

“We’ll never abandon our prisoners. We’ll never leave them [as] trophies to the Zionist sadism and they won’t remain much longer behind the bars of the occupation,” Haniyeh said outside the Gaza headquarters of the Red Cross, according to the Walla news site.

Haniyeh’s warning to Israel over Arbid joined that of the Popular Front for the Liberation of Palestine terror group, which Israeli authorities said the suspect had a history of involvement with.

“We hold the Israeli occupation responsible for the lives of the Palestinian prisoner Samer Arbid and his comrades, and we assure that by endangering their lives, the occupation will open the gates of hell,” the PFLP’s military wing said in a statement Sunday.

Arbid’s lawyers have called for an international probe into the incident to be conducted by the United Nations and the International Red Cross.

Arbid was arrested earlier this month and then released due to lack of evidence against him. He was picked up again on Wednesday.

The Shin Bet was given legal permission to employ “extraordinary measures” in its interrogation of Arbid, a security source told reporters Saturday. Such measures can include beatings, forcing prisoners into uncomfortable positions, sleep deprivation, shackling and subjecting prisoners to extreme temperatures.

This is typically allowed in cases of a “ticking time bomb,” where there is concern that the suspect could provide security forces with information that could prevent an imminent attack.

Over the weekend, the Shin Bet announced the arrest of three Palestinians suspected of carrying out a deadly bombing attack last month at a natural spring outside the Dolev settlement in the West Bank.

The Shin Bet said the cell was planning additional attacks when the suspects were arrested, including shootings and kidnappings. During the arrest raids, security forces also found and safely detonated an improvised explosive device that the group had made.

Arbid was taken to Jerusalem’s Hadassah Hospital at Mount Scopus in critical condition after he apparently suffered a heart-related problem during the interrogation.

His attorneys said he had been healthy when he was arrested. The Shin Bet confirmed only that Arbid had been taken to the hospital.

On August 23, an IED that had been planted next to the Bubin natural spring in the central West Bank was triggered by terrorists as the Shnerb family from the central Israeli town of Lod visited the site. Rina Shnerb was declared dead at the scene and her father Eitan and brother Dvir, 19, were taken to a hospital in Jerusalem after being wounded by the blast.

Both Qassem a-Karim Rajah Shibli and Yasan Hasin Hasni Majamas, who were also arrested over their suspected involvement in the attack, have served time in Israeli prisons for their involvement in terrorist activities, according to the Shin Bet.

The Shin Bet said it arrested a fourth man — Nizam Sami Yousef Ulad Mahmoud, 21 — who is suspected of being a member of Arbid’s cell. He is a member of the PFLP’s student group at Bir Zeit University in the West Bank.

The Shin Bet said it was still looking for additional members of the terror cell.

Jacob Magid and Judah Ari Gross contributed to this report.