The attack triggered what President George W. Bush’s administration called its war on terror, prompting the U.S. military invasions of Afghanistan and Iraq and years of US operations against armed extremist groups elsewhere in the Middle East.

The attack and US retaliation brought the overthrow of two governments outright, devastated communities and countries caught in the battle, and played a role in inspiring the 2011 Arab Spring popular uprisings against authoritarian Middle East governments.

At home, the attacks inspired a sharply more militaristic and nationalist turn to American society and culture.

US authorities point to Mohammed as the source of the idea to use planes as weapons. He allegedly received approval from al-Qaeda leader Osama bin Laden to craft what became the 9/11 hijackings and killings. US forces killed bin Laden in 2011.

Authorities captured Mohammed in 2003. Mohammed was subjected to waterboarding 183 times while in CIA custody before coming to Guantanamo, along with other torture and coercive questioning.

The use of torture has proven one of the most formidable obstacles in US efforts to try the men in the military commission at Guantanamo, owing to the inadmissibility of evidence linked to abuse. Torture has accounted for much of the delay of the proceedings, along with the courtroom’s location a plane ride away from the United States.

Daphne Eviatar, a director at the Amnesty International USA rights group, said she welcomed news of some accountability in the attacks.

She urged the Biden administration to close the Guantanamo Bay detention centre, which holds people taken into custody in the so-called war on terror. Many have since been cleared, but are awaiting approval to leave for other countries.

Strada, national chairperson of a group of families of victims called 9/11 Families United, had been at Manhattan federal court for a hearing on one of many civil lawsuits when she heard news of the plea agreement.

Strada said many families have just wanted to see the men admit guilt.

“For me personally, I wanted to see a trial,” she said. “And they just took away the justice I was expecting, a trial and the punishment.”

Michael Burke, one of the family members receiving the government notice of the plea bargain, condemned the long wait for justice, and the outcome.

“It took months or a year at the Nuremberg trials,” said Burke, whose fire captain brother Billy died in the collapse of the World Trade Center’s North Tower. “To me, it always been disgraceful that these guys, 23 years later, have not been convicted and punished for their attacks, or the crime. I never understood how it took so long.”