Spain says it has evidence against the suspects
The renewed detention of eight men in Spain over the weekend on suspicion of taking part in planning the 11 September attacks on the United States marks the latest stage in a worldwide investigation.
The biggest ever criminal investigation has produced countless leads but, as yet, few real breakthroughs.
As well as the 1,000 or so people detained in the US - most of them for suspected immigration irregularities - there have been numerous arrests throughout Europe and other countries, including Malaysia, Thailand and the Philippines.
But no one has yet been detained or charged with direct involvement in the attacks, which left nearly 5,000 people dead.
Documentary evidence
The difference with the arrests in Spain is that the investigating judge believes those being detained took part in preparations for the September hijackings, rather than in other alleged terrorist activities.
The eight men deny the allegations.
But Spanish authorities say they have documentary evidence and recordings of telephone conversations to back them up.
They also say they have established a link between the suspects and Mohammed Atef, a senior al-Qaeda figure reportedly killed by US bombing in Afghanistan last week.
The main focus of investigations in Europe had been the German city of Hamburg, where three of the suspected hijackers attended university before moving to the United States.
German police and American FBI agents are searching for three others who were based in Hamburg and are believed to have helped plan the attacks.
In Britain, another suspect accused of providing flight training for the hijackers is due to face an extradition hearing next week.
For all the work of the investigators, though, there is so far little to show.