Madrid, Spain ? After an eight-year investigation, Spanish prosecutors opened Europe’s biggest trial of al-Qaida suspects Friday in a case that includes three defendants accused of playing a supporting role in the Sept. 11, 2001, attacks.

Spanish authorities imposed extra security for the trial, including a retrofitted courthouse designed especially for terrorism cases. Police helicopters and guards with machine guns patrolled the grounds. Inside the courtroom, all but one of the two-dozen defendants sat on benches inside a large bulletproof-glass cage that isolated them from their lawyers, prosecutors and the three-judge panel hearing the case.

Prosecutors say the suspects were part of a Spain-based cell of al-Qaida followers who raised money and recruited fighters for radical Islamic causes in Bosnia, Afghanistan and Indonesia. Most face charges of financing terrorism and belonging to a terrorist organization, but three are specifically accused of assisting two of the ringleaders of the 9-11 attacks by organizing a rendezvous in a Spanish coastal town two months prior to the hijackings.

Spanish investigators amassed 300 boxes of evidence and an estimated 100,000 pages of documents, which were stacked along one wall in the courtroom. Much of the evidence in the case is circumstantial, and each defendant has asserted his innocence.

The challenges facing prosecutors became apparent during questioning of the first witness, Luis Jose Galan, a Spanish convert to Islam who faces up to 18 years in prison for allegedly belonging to al-Qaida and possessing weapons illegally.

Court officials say the trial is likely to last at least four months.