The US government has described it as the "most egregious criminal act on American soil in modern history".

But a quarter of a century on, the trial of its alleged mastermind is yet to begin and families of those killed fear that time for justice to be delivered is running out.

Tom Resta's brother John and his wife Sylvia, who was seven months pregnant, were among the almost 3,000 people killed on 11 September 2001 when hijackers flew planes into the World Trade Center in New York City and the Pentagon in Virginia. Another crashed in a field in Pennsylvania after the passengers fought back.

The couple had been preparing their unborn baby's nursery, painting it with characters from the children's television show Blue's Clues in the days before 9/11. They had dreamed of having a baby and doted on the children of friends and family.

On the day of the attacks, the couple - both traders - were working on the 92nd floor of the north tower of the World Trade Center.

"After all this time, it still chokes me up," Resta told the BBC through tears. While 25 years have passed, he said 9/11 was "almost constantly" on his mind.

Resta has been to the US naval base of Guantanamo Bay four times to watch complex pre-trial proceedings for alleged 9/11 mastermind Khalid Sheikh Mohammed and his co-defendants - a process he describes as "laborious and tedious".

A military prison was established on the base in south-eastern Cuba during the "war on terror" that followed the 9/11 attacks, and Mohammed - often referred to as KSM - has now been held there for 20 years without ever being convicted.

Relatives of victims enter a lottery process to attend the proceedings in a high-security courtroom, where thick glass separates them and members of the press from the defendants.

The pre-trial process has been going on for so long that it has now been presided over by five different military judges.

Last month, as the 25th anniversary approached, there was a flurry of movement and something of a breakthrough, with a trial date set for Mohammed for June 2028.

But Resta said he was bracing himself for further delays, and with so much time having passed, some of his relatives feared they would not live long enough to see the trial.

"My father will be 98 in November, and I don't think he's expecting to see it. I have a lot of family members - aunts, uncles - that would all like to see it happen. But my parents in particular, I don't think they expect to live long enough," he said.

"Hopefully all the defendants will still be alive by the time we get to trial. That's a big worry," he added.

A tribute in light above the New York City skyline, at the 2024 anniversary

Resta's concerns that a trial could still be further delayed were shared by relatives of other 9/11 victims.

Earlier this year, Stephan Gerhardt made his sixth trip to Guantanamo. He said seeing convictions for the death of his younger brother Ralph was vital for him, but something he wished he no longer had to think about.

"I want to concentrate on the memories," he said. ?"I still have Ralph's car. I'm trying to restore it and get it in pristine shape again… We were recently visiting my parents, and we found some of his things that they were holding onto, and the emotion behind it is like, 'yeah, this is just some old stuff, but it belonged to my little brother'."

Gerhardt said his "biggest concern" was that he wanted to see Mohammed and his alleged co-conspirators convicted before they died.

"They're getting elderly… I don't want them to pass away without being convicted, because that would be an injustice to all the family members - that we captured these people, but we never got to try them and convict them for the mass murder that they did of our loved ones."

Khalid Sheikh Mohammed as he appeared on the FBI's Most Wanted website

Mohammed has been charged with offences including conspiracy and murder, with 2,976 victims listed on the charge sheet. He is accused of conceiving the idea of training pilots to fly commercial planes into buildings and taking those plans to Osama bin Laden, leader of the militant Islamist group al-Qaeda.

He has previously said he planned the "9/11 operation from A to Z".

But days after the trial date was set for 2028, the military judge threw out confessions made by Mohammed to FBI agents on Guantanamo Bay after he was brought there from secret CIA prisons known as "black sites", which he had been held in for years. In his ruling, the judge described Mohammed's treatment by the CIA as "extraordinary physical and mental abuse".

He said the confessions were not voluntary and could not be used against Mohammed at trial.

Experts say the torture of Mohammed and his alleged co-conspirators has been the main reason for the lengthy delays in the case.

"Every single issue has been litigated through the issue of torture," said Kasey McCall-Smith, an international law professor at Edinburgh University.

She said the defendants were subjected to "years of abuse", including waterboarding and stress positions.

John and Sylvia Resta on their wedding day

John Ryan, author of America's Trial: Torture and the 9/11 Case on Guantanamo Bay, said last month's ruling means prosecutors lose what they had described as their best evidence against Mohammed, but it is unclear how much it will impact the case.

The prosecution has other evidence, Ryan says, including intercepted phone calls and secret recordings, but whether they can feature in the trial is not yet guaranteed. "Mohammad's team and the other defence teams will file motions to suppress some of these categories of evidence, as well."

During the George W Bush administration, the US admitted using "enhanced interrogation techniques" to acquire information from suspects but said these methods saved lives and never amounted to torture. President Barack Obama banned them on his second day in office.

President George W Bush on Air Force One after the attacks

Relatives of the victims say the torture of the defendants has so far blocked their right to justice.

"If we would've not tortured them, the legal implications here would've been much, much simpler. They probably would've been sentenced to death a long time ago," said Gerhardt. "I can't control what the government did, and we've got to live with the impacts."

Both Gerhardt and Resta travelled to Guantanamo in early 2025 when Mohammed and two of his co-defendants had been due to plead guilty under a controversial deal struck with US government prosecutors, through which they would not face a death penalty trial.

The BBC was with them at the base when the pleas were paused at the last minute after the US government argued that to "deprive the government and the American people of a public trial as to the respondents' guilt and the possibility of capital punishment" would cause "irreparable" harm.

Legal wrangling over the validity of the deal is continuing.