Official White House Photo by Adam Schultz

(WASHINGTON) — U.S. President Joe Biden has rejected proposed conditions for a plea deal for five Guantanamo Bay detainees accused of aiding in the Sept. 11, 2001, terrorist attacks, according to a spokesperson for the White House National Security Council.

The defendants had put forward a set of demands as a basis for plea negotiations, known as the “joint policy principles.” According to The New York Times, those demands include avoiding solitary confinement and receiving health treatment for injuries the detainees claim were a result of CIA interrogation methods.

Biden agreed with U.S. Secretary of Defense Lloyd Austin’s recommendation not to accept those demands.

“The 9/11 attacks were the single worst assault on the United States since Pearl Harbor,” the National Security Council spokesperson told ABC News in a statement on Wednesday. “The President does not believe that accepting the joint policy principles as a basis for a pre-trial agreement would be appropriate in these circumstances. The Administration is committed to ensuring that the military commissions process is fair and delivers justice to the victims, survivors, families, and those accused of crimes.”

The five detainees, including the alleged mastermind behind the attacks, Khalid Shaikh Mohammed, were transferred to the U.S. detention center in Guantanamo Bay, Cuba, in 2006. Their case has been held up by legal proceedings for years, with no trial date set.

A lawyer who represents one of the detainees told ABC News on Wednesday that they cannot share the full list of joint policy principles rejected by the Biden administration, but said the list focused on improvements in long-term conditions of confinement to include a comprehensive torture rehabilitation program.

Ultimately, it will not be the president’s decision to determine if there is a pre-trial agreement or what an appropriate sentence would be. Rather, that falls to the Office of the Convening Authority for the Office of Military Commissions, a branch of the U.S. Department of Defense that handles cases in the military court of law.

ABC News has reached out to the Office of Military Commissions for comment.

The Office of the Chief Prosecutor for Military Commissions sent a letter, dated Aug. 1, to families who lost loved ones in the 2001 attacks, notifying them that a plea deal was being considered that could take the death penalty off the table.

When asked for comment about the plea deal, Office of Military Commissions spokesperson LCDR Adam Cole told ABC News last month: “We can confirm that the Department of Defense’s Office of the Chief Prosecutor, specifically prosecutors assigned to prosecute those alleged to have been involved in the planning and execution of the 9/11 attacks, released a letter to 9/11 victims’ family members on Aug. 1. The letter described potential outcomes of what a potential plea agreement may mean, including the possibility of removing the death penalty. Negotiations for a fair and just conclusion to this case are ongoing and have not been concluded. The Office of the Chief Prosecutor uses these letters as well as in-person and virtual meetings to ensure that victims’ family members remain up to date—and can provide feedback. Office of the Chief Prosecutor remains committed to hearing and considering victim and family member input on any agreement prior to the conclusion of negotiations and continues to utilize a variety of methods and opportunities for victims and family members to offer their perspectives on the pending military commissions cases.”

On the morning of Sept. 11, 2001, two hijacked passenger jets flew into the twin towers of the World Trade Center in New York City, marking the start of a series of coordinated attacks that day against the United States by the Afghanistan-based terrorist group al-Qaida. Nearly 3,000 people were killed that day and thousands more were injured.

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