Five men accused of directing and financing the Sept. 11 plot were back in their high-security cellblock at the Guantanamo Bay detention center after a pretrial hearing when the general in charge of prosecuting them delivered a somber message for families of people killed in the attack.

"I pledge to you that the nation will never forget, will never lose interest," Army Brig. Gen. Mark Martins said last week to a handful of reporters who traveled to this U.S. base to view the latest proceedings. "And your government will continue to pursue justice under law for however long that takes."

It could be a very long time. The prosecution of the "9/11 five" has been unfolding incrementally at a courtroom built specifically for the proceedings on an abandoned airfield. And it is likely to be years before Khalid Shaikh Mohammed, the al-Qaida commander who has claimed responsibility for the Sept. 11, 2001, attack, and his four alleged co-conspirators go before a jury of military officers.

The four days of hearings that ended last Friday was the 14th round of pretrial sessions since the May 2012 arraignment on charges that include nearly 3,000 counts of murder in violation of the law of war for deaths in New York, Washington and Pennsylvania on Sept. 11. The case is often portrayed as the most complex and expensive terrorism prosecution in U.S. history. It may ultimately be the longest.

That reality has sparked anger and frustration among some observers, including the small groups of victims' relatives chosen by lottery to attend proceedings.

More often, the reaction is like that last week of John Olson from Rockville Centre, New York, who is simply resigned to the situation.

"These guys aren't going anywhere," said Olson, whose wife, Maureen, was killed at her job on the 96th floor of the North Tower of the World Trade Center. "Quite frankly, if they stay here the rest of their lives that wouldn't bother me."

There are a number of inter-related reasons the case has advanced slowly. Mostly, though, it stems from the fact that the defendants face trial at Guantanamo amid a broader debate over whether to close the detention center and that they are being tried by military commission, a hybrid system that has been used throughout U.S. history to prosecute battlefield offenses during war time but never for a case like this one.

The defendants were transferred in September 2006 to Guantanamo from CIA detention facilities, where they were subjected to what the government called "enhanced" interrogation, now widely regarded as torture. President George W. Bush directed they be tried by military commission and they were arraigned about 18 months later.

The case was still in the pretrial stage when President Barack Obama took office pledging to close the Guantanamo jail. He sought to move the case to a civilian court in New York, but ran into intense political opposition. The administration backtracked and sent the case back to a military commission with some added legal protections for the accused, and they were arraigned for a second time 3½ years ago. Congress later prohibited sending any Guantanamo detainees to the U.S. for any reason, including trial.

The Guantanamo commissions, which combine elements of the civilian and military justice systems, are the first held by the U.S. in decades and the law has evolved in those years, meaning many basic issues must be litigated.

The court, presided over by an Army colonel who has handled some of the most prominent recent U.S. military trials, has wrestled with what elements of the Constitution apply to defendants held at Guantanamo or whether a defendant can represent himself, issues long resolved in civilian courts. Last week, the judge finally considered but didn't rule on a defense motion filed days after their arraignment that the charges should be dismissed because statements by Obama, Bush and other senior officials about the defendants may have improperly swayed proceedings.

Many of these matters are being argued with an eye to later appeals that are likely to last years, and ultimately reach the Supreme Court.

"There are so many issues around the structure of the military commissions and around discovery that there's almost an incalculable amount of work to be done, to work our way through all these problems," David Nevin, a civilian lawyer for Mohammad, said in an interview.

The government is barred by law from using evidence obtained by coercion during the defendants' time in CIA prisons. Prosecutors are expected instead to rely on statements the defendants made to the FBI and others at Guantanamo. Lawyers for the defendants say anything the men said while in custody is tainted by their harsh treatment, a contention that will likely form the core of the defense in either the trial or the penalty phase if they are convicted.

Issues surrounding the secrecy, and the government's desire to protect classified information about their time in CIA custody, are responsible for much of the slow movement. Proceedings came to a near halt for 18 months while the judge tried to determine if an FBI investigation into potential security breaches on one of the five defense teams created a conflict of interest. The case also detoured when the defense discovered microphones disguised to look like smoke detectors inside meeting rooms and when several defendants recognized a new Arabic interpreter as someone they encountered at a secret CIA prison.

Defense lawyers routinely deny they are seeking to delay a trial for men who could get the death penalty if convicted. Walter Ruiz, who represents Saudi defendant Mustafa al-Hawsawi, said he wants to get on with it to prove his client was at most a minor participant in the plot.

"He has been in captivity since 2003," Ruiz said. "At some point there is a desire for finality."

Martins, the general in charge of the prosecution, said that despite frequent stalls there is "methodical" movement toward trial, although he will no longer predict a date for its start. It's an adversarial system by design. "That's the system we set up. It is not driven by a timeline," he said. "I understand people's frustrations. I hear it all the time and deal with it. "

Prosecutors are working seven days a week to review classified files to determine what they must turn over to the defense and expect to finish that effort by Sept. 30, 2016, Martins said.

That may not be the end of it. Ruiz predicts there will be years of litigation over evidence discovery.

Looking ahead to a possible trial date, he said, "I think 2020 would be optimistic."