Barghouti has been imprisoned for more than 20 years, and was convicted of murder in 2004. A leader of the second intifada, Barghouti, now 64, received five life sentences, and Israeli officials said he was a terrorist who ordered suicide bombings against civilian targets. Barghouti, who said in court he had no connection to the attacks, is being held in the Megiddo Prison, a maximum-security facility.

Securing Barghouti’s release is a popular cause among Palestinians who see him as a possible successor to Palestinian Authority leader Mahmoud Abbas. Barghouti has been near the top of Hamas’s list of prisoners it wants Israel to release in exchange for Israeli hostages held in Gaza, according to two Middle Eastern officials who are familiar with the negotiations to secure the release of hostages and a cease-fire. The officials spoke on the condition of anonymity to discuss sensitive diplomacy.

In an interview with The Washington Post, Barghouti’s son, Arab, who is based in the West Bank, said that after Oct. 7, when Hamas attacked southern Israel and killed about 1,200 Israelis, his father was physically assaulted, placed in solitary confinement in darkness for 12 days, and the Israeli national anthem was pumped into his cell “at a very high volume, from around 5 a.m. until midnight, for many days.”

A lawyer who met with Barghouti this week said in a written report to the Barghouti family that he saw bruising over his right eye and that Barghouti showed him bruising on his back and right foot. The lawyer wrote that Barghouti told him that, on March 6, “I was beaten for long minutes all over my body, mainly on the face, back, and legs. The severity of the beating caused me to collapse to the ground, at which point they persisted in striking me until I lost consciousness.”

The Post reviewed a copy of the report. Barghouti’s lawyers declined to comment.

A spokesperson for the Israeli Prison Service said the service “is a law-abiding organization. We have no knowledge of these claims.”

The State Department has not named Barghouti in public statements, but U.S. officials said they are aware of allegations of abuse. The department said in a statement to The Post that it has informed Israel that it must “thoroughly and transparently investigate credible allegations of and ensure accountability for any abuses or violations.” The statement, which the department issued in response to questions about Barghouti, said that Palestinian detainees must be held in “dignified conditions and in accordance with international law.”

Itamar Ben Gvir, the far-right minister of national security in the government of Israeli Prime Minister Benjamin Netanyahu, has since the Oct. 7 attacks barred the International Committee for the Red Cross, as well as family members, from visiting the roughly 9,000 Palestinian prisoners and security detainees in custody of the Israeli Prison Service. The ICRC’s last publicly recorded visit with Barghouti was in November 2017, months after he finished a 24-day hunger strike.

“We have continuously asked to resume our visits, as they are central not just to our mission in Israel and the Occupied Territories but because they are a lifeline for the detainees and their family,” said Jessica Moussan, a spokeswoman for the ICRC.

During a visit to Israel on Friday, Secretary of State Antony Blinken told Israel’s war cabinet that it should allow the ICRC access to detainees, said a senior State Department official, who spoke on the condition of anonymity to discuss Blinken’s closed-door discussions. Israeli rights groups submitted a Supreme Court petition to the same end on Feb 24.

An Israeli diplomatic official, who spoke on the condition of anonymity to discuss the matter, denied any knowledge of the mistreatment of Barghouti: “We have laws in our country; such behavior is not permitted and won’t be condoned if indeed proven to be true.”

The two Middle Eastern officials, who represent different governments, said they had received reports about the worsening situation of some Palestinian prisoners, including Barghouti, at the same time they were receiving reports about the harsh treatment of Israeli hostages in Hamas captivity.

“It was communicated to the Israeli government that any bad treatment of Palestinian prisoners would give Hamas a bigger excuse to treat Israeli hostages worse than they are being treated already,” said one of the officials. “It’s a vicious circle.”

Sen. Chris Van Hollen (D-Md.), a member of the Senate Foreign Relations Committee, said the United States should specifically warn Israel about its treatment of Barghouti.

“The Biden administration should make it very clear to the Netanyahu government that if Barghouti is harmed or killed in prison, it would throw gas on a raging fire,” he said in an interview. “Barghouti is probably the most popular Palestinian leader. That’s true in the West Bank and in Gaza.”