The start of Europe's largest trial of al-Qaeda members is the result of a long investigation led by renowned Spanish judge Baltasar Garzon.
Yarkas allegedly became Spain's al-Qaeda leader in 1995
Mr Garzon began his inquiry into Spain's emerging Islamist network five years before the 11 September 2001 attacks that hit the World Trade Center and the Pentagon in the US.
Two months after the attacks, the first arrests were made - and his investigation took on a new sense of urgency.
By September 2003, his 692-page-long findings listed Osama Bin Laden among the 35 people charged with terror-related offences, including the 11 September plot. The list was later expanded to include 41 individuals.
Bin Laden remains at large and Spanish law does not allow for trial in absentia.
But the 24 people in the dock in April 2005 included the alleged leader of the Spanish al-Qaeda operation - Syrian-born Immad Eddim Barakat Yarkas, 42, also known as Abu Dahdah.
Mr Yarkas and two other suspects - Driss Chebli and Ghassub al-Abrash Ghaylun - face charges of direct involvement in the 9/11 plot, including the murders of the 2,500 people who died.
The other suspects are accused of al-Qaeda membership. All the defendants deny the charges.
Mr Yarkas is accused of leading Spain's al-Qaeda cell since 1995.
A Spanish citizen who describes himself as a businessman dealing in second-hand cars, Mr Yarkas was arrested in November 2001, about four years after Spanish police began tapping his telephone.
His number was found by German police in the Hamburg flat believed to have been used by two of the hijackers - including Mohamed Atta, the pilot of one of the planes that hit the Twin Towers. Atta's phone number was also found in Mr Yarkas's European contacts, said the Spanish judge.
In his indictment of 17 September 2003, Judge Garzon outlined how Spain had been used as a staging ground for the 11 September 2001 attacks.
This was believed to have started with a trip to the US in 1997 by one of the three top suspects in the current trial, Syrian-born Ghassub al-Abrash Ghaylun, with the purpose of filming the World Trade Center and other buildings seen as potential targets.
The tapes were allegedly passed on to "operative members of al-Qaeda and would become the preliminary information on the attacks against the Twin Towers", wrote Judge Garzon.
Two weeks before the 11 September attacks, Mr Yarkas received a phone call in which a man called "Shakur" - later identified as Farid Hilali - told him in cryptic language: "We've entered the field of aviation, and we've even cut the throat of the bird."
Spain believes the 9/11 plot was finalised there
This was taken to be a reference to the final stage in preparing the attacks - even though the US commission that investigated the 11 September events found no evidence to link "Shakur" to the plot.
According to the indictment, Mr Yarkas also provided cover and financial support for a key meeting in July 2001 in the Spanish town of Tarragona.
The meeting was between Mohamed Atta and Ramzi Binalshibh, another top 9/11 suspect who has been in US custody since being arrested in Pakistan in September 2002.
The third defendant accused in Spain over the 11 September attacks, Moroccan-born Driss Chebli, was also said to have been instrumental in setting up the rendezvous - believed by US and European officials to have been called to set a date for the attacks.
The US 9/11 Commission report said it had seen no evidence that any individuals based in Spain took part in the sJuly meeting "or in the 9/11 plot".
The others 21 suspects appearing in court on Friday include a journalist from the Arabic TV station al-Jazeera, Tayseer Alouni, who interviewed Bin Laden after the attacks.
He is accused of using a posting in Afghanistan to distribute money to the militant Islamic network.
Mr Yarkas' lawyer, Jacobo Teijelo, said Spanish prosecutors "have no solid evidence of anything".
A former aide to Spain's prime minister at the time of the attacks also said the proof against the suspects was "legally quite weak," the Washington Post newspaper reported.
"We have a tremendous gap between what we think they did and the ability to prove them guilty," said Rafael Bardaji.