Brett Eagleson, president of 9/11 Justice, an organization representing 9/11 survivors and families of the victims, said in a statement to the BBC that the families were “deeply concerned about these plea bargains”.
He said the process lacked transparency and urged authorities to gather more information about Saudi Arabia's role in the attack.
“I was heartbroken to hear today that there was a plea deal that gave the detainees at Guantanamo Bay what they wanted,” Terry Strada, whose husband Tom has died, told the BBC's Today Programme.
“This is a victory for Khalid Sheikh Mohammad and the other two men, and their own victory,” added Ms. Strada, national chairwoman of the 9/11 Families Coalition campaign group.
The 9/11 attacks were the deadliest attack on U.S. soil since the Japanese attack on Pearl Harbor in Hawaii in 1941, killing 2,400 people.
The plea agreement was first announced in a letter prosecutors sent to the victims' families, according to The New York Times., Out.
He said arguments at the military court could begin as early as next week.
“The specific terms of the pre-trial agreement have not been made public at this time,” the Pentagon said in a statement.
They are charged with a number of charges, including attacks on civilians, murder under the laws of war, kidnapping and terrorism.
Khalid Sheikh Mohammed is widely believed to be the architect of the attacks in which hijackers crashed passenger planes into the World Trade Center in New York and the Pentagon outside Washington.
A fourth plane crashed in a field in Pennsylvania after passengers resisted.
Mohammed, an American-educated engineer, was arrested in Pakistan with Hausawi in March 2003.
Prosecutors alleged he shared his ideas about hijacking planes and attacking U.S. buildings with al Qaeda leader Osama bin Laden, and later helped recruit and train some of the hijackers.
He was subjected to various “enhanced interrogation techniques,” including waterboarding, at least 183 times before the U.S. government banned the practice.
One reason the trial has been so long delayed is because of concerns that so-called brutal interrogation techniques, which critics say amount to torture, could tamper with evidence against the detainees.
“This is the least bad deal that could happen in the real world,” national security analyst Peter Bergen told CNN.
In September, the Biden administration reportedly rejected plea bargaining terms for five people detained at a U.S. naval base in Cuba, including Mohammed.
The men reportedly asked the president to ensure they would be given trauma treatment rather than being held in solitary confinement.
The White House National Security Council said Wednesday that the White House had been informed of the new terms of the deal but had no role in the negotiations.
