WASHINGTON — Khalid Sheikh Mohammed, accused as the main plotter in al-Qaeda’s Sept. 11, 2001, attacks on the United States, has agreed to plead guilty, the Defense Department said Wednesday, pointing to a long-delayed resolution in an attack that altered the course of the United States and much of the Middle East.

He and two accomplices, Walid Bin Attash and Mustafa al-Hawsawi, are expected to enter the pleas at the military commission at Guantanamo Bay, Cuba, as soon as next week.

Pentagon officials declined to immediately release the terms of the plea bargain. The New York Times, citing unidentified Pentagon officials, said the terms included the men’s longstanding condition that they be spared risk of the death penalty.

The U.S. agreement with the men to enter into a plea agreement comes more than 16 years after their prosecution began for al-Qaeda’s attack. It comes more than 20 years after militants flew commandeered commercial airliners into buildings. The attack killed nearly 3,000 people and triggered years of U.S. wars against militant extremist groups that reshaped Middle East countries and, in many ways, U.S. society.

Terry 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.”

“They were cowards when they planned the attack. And they’re cowards today,” she said.

Dozens of relatives of those killed died while awaiting resolution of the case, Strada said.