One day after federal prosecutors announced that they had struck a plea deal with three men accused of plotting the Sept. 11, 2001, attacks, New Yorkers and family members of victims had mixed reactions to the sudden news. Under the agreement, the three men — including Khalid Shaikh Mohammed, the accused mastermind — will serve life in prison but avoid a death-penalty trial.

“If Khalid Shaikh Mohammed doesn’t deserve the death penalty, who does?” said Don Arias, a retired Air Force lieutenant colonel, whose brother Adam was killed in the World Trade Center. From his home in Florida, Mr. Arias, 67, remembered days of searching for his brother before his remains were identified.

Mr. Arias was upset about the plea deal, and especially about the sense that people seemed to have lost interest in an act of terrorism that claimed nearly 3,000 lives and shattered Americans’ sense of security.

“Most people I talk to figure this was done years ago,” he said, referring to the trial. “My brother was 37 years old. It hurts to know that someone I know and love is now all but forgotten.

“After 9/11, we all said, ‘Never forget.’ Well, we forgot. And not only did we forget, we don’t give a damn anymore. A lot of people just want this over with.”

At the 9/11 Memorial and Museum in Lower Manhattan on Thursday, many people seemed unaware of the developments.