New York, New York - A proposal to allow alleged perpetrators of the 9/11 attacks to plead guilty and avoid the death penalty poses a powerful dilemma for victims' families, some of whom still want to seek the ultimate retribution after two decades of legal limbo.

The proposal detailed by prosecutors in a letter this month could offer families of the nearly 3,000 victims the best path to a resolution of a case bogged down in pre-trial maneuverings in the Guantanamo military commissions for years – and with no end in sight.

But some families of those killed in New York's World Trade Center, the Pentagon, and in Pennsylvania say a deal without a trial could mean the entire truth about what happened on September 11, 2001, might never be told.

Others say that every year of delay means that more people pass away without seeing justice for their slain relatives – and increases the risk that the aging defendants themselves could die without ever being found guilty.

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"All 9/11 family members want justice and accountability. Too many of us have died in the last two decades without either," said September 11th Families for Peaceful Tomorrows, which backs the plea deal.

"Plea agreements, which could be made right now, would offer finality: an end to the 9/11 military commission, clear admissions of guilt, and life sentences without parole or any possibility of appeal," they said.

But Dennis McGinley of the group 9/11 Justice said the deal would leave untold the full story behind the attack that killed his brother Danny in the south tower of the World Trade Center.

"All this is, is... to prevent a trial from taking place where Khalid Sheikh Mohammed is going to have to spill the beans," he said, referring to the self-described 9/11 mastermind also known as "KSM."

The deal, outlined in an August 1 letter from the office of the chief prosecutor for the Pentagon-run military tribunals, has been in preparation for two years in the case of KSM, Ammar al-Baluchi, Walid bin Attash, Ramzi bin al-Shibh, and Mustafa al-Hawsawi.

Each has been held for more than 16 years at the US naval base in Guantanamo Bay, Cuba, where they are among the last 30 of what was once nearly 800 people detained extrajudicially by the United States after 9/11.

They were formally arraigned in 2012 for the case, but since the beginning, it has been mired in debates over prosecutors' intent to use evidence that defense attorneys say was extracted through systematic torture at the hands of the CIA.

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The letter implicitly acknowledges that prosecutors cannot say when a full trial would begin, if ever.

In the proposed deal, the accused "would accept criminal responsibility for their actions and plead guilty to the charged offenses in exchange for not receiving the death penalty," the letter said.

It said the defendants would have to agree to a "stipulation of facts," which would provide details of the September 11 plot and their roles in it.

While the prosecutors said no deal was finalized, the letter was confirmation that such an arrangement appears to be where the case is heading.

Indeed, the prospect of more delays sharpened last week when a military judge in a separate Guantanamo case rejected torture-tainted confessions.

Dropping the death penalty in the case, though, could spark an emotional backlash not only from victims' families but from Americans across the country, where anger remains deep over the Al-Qaeda attack.

"Khalid Sheikh Mohammed and the other 9/11 perpetrators should never be given a plea deal and should face the full measure of justice for their actions – the death penalty," said New York Congressman Mike Lawler, criticizing President Joe Biden for the deal.

But Terry Rockefeller, of the Peaceful Tomorrows group, said the deal is the best chance for families to get closure, with the certainty that defendants will have no ability to appeal.

Rockefeller, whose sister died in the World Trade Center, said that in private meetings earlier this year, prosecutors had no answer when asked how long a trial might take.

"The military commissions have by and large been a failed system," she said, adding, "No trial is going to lead to a death penalty because of the torture issue."

McGinley said the type of punishment didn't matter to him at this point.

"Whether it's the death penalty or life in prison, I want whatever the terrorists don't want," he said.

But he argued that a settlement will allow the government to keep information classified about the attacks – information he said would implicate Saudi Arabia more deeply in the carnage.

Riyadh has long denied any links with the hijackers, but suspicions have lingered over whether the attacks were funded with Saudi money, and McGinley’s group alleges that Saudi agents were involved in various ways.

He thinks a trial – with all evidence declassified – would be the best chance for families to get justice after two decades of waiting.

"The 9/11 community has been getting used and abused by our government for the last 22 years. At this point, it's almost cruel and unusual punishment," he said.