The five men who are accused of conspiring in the Sept. 11, 2001, attacks were captured two decades ago, but a trial is still unlikely to start before 2026.

Instead, the death penalty case has been delayed repeatedly.

An entire generation of Americans has no memory of the attacks, which killed nearly 3,000 people, or the creation of the Pentagon prison and court at Guantánamo Bay in Cuba. Critical witnesses have died, and others have fading memories.

Prosecutors say the defense lawyers are trying to derail the trial or avert the death penalty by exploring every possible avenue in the case. Defense lawyers have accused prosecutors of using national security concerns to prevent turning over evidence.

Both are true. But more fundamental decisions and problems stand in the way of a trial.

The government wanted to try the men together.

The five men were charged together, meaning an issue involving one would dictate the pace of the trial for the others. The lead defendant, Khalid Shaikh Mohammed, is accused of being the mastermind of the plot and proposing it to Osama bin Laden. Two other men are accused of helping some of the hijackers with finances and wire transfers.

For years, prosecutors were devoted to the idea of a one-and-done trial rather than trying the men separately. In 2023, however, a medical panel concluded that one defendant, Ramzi bin al-Shibh was not competent to stand, and a judge severed his case.

Now the case has four defendants, each with his own team of lawyers.

Three of them, Mr. Mohammed, Mustafa al-Hawsawi and Walid bin Attash, negotiated an agreement with prosecutors to plead guilty in exchange for life sentences. But Defense Secretary Lloyd J. Austin III rescinded it.