The US Justice Department originally took more than 1,100 people into custody but almost nothing is known about who they are or why they were detained, except that they are mainly of Middle East origin. Some of them are beleievd to have been cleared and released. The list of suspected terrorist organisations - whose members or supporters are barred from entering the US - has grown steadily. It includes charitable organisations and food companies accused of funnelling money to al-Qaeda. Evidence can be as limited as buying airline tickets on the internet, having a Saudi name and a history of large money transfers from the Middle East.
Zacarias Moussaoui, 33, is the first person to be indicted for conspiring with Osama Bin Laden and other suspects to kill thousands in the 11 September attacks.
A French citizen of Moroccan descent, Moussaoui was detained on immigration charges in August 2001 when he aroused suspicion at a Minnesota flight school where he sought training. He is reported to have said he wanted to learn how to fly planes but was not interested in how to take off and land them.
Moussaoui is alleged to have received training in al-Qaeda camps in Afghanistan, sought pilot training and to have received funding from sources in Germany and the Middle East.Before 2001 he lived in London, where he did a Masters degree at South Bank University. From 98-99 he lived in Hamburg, where he shared a flat with Mohammed Atta.
Moussaoui is accused of being involved in the planning of the 11 September attacks. He has been indicted on six counts of conspiring with the al-Qaeda leader, Osama Bin Laden; those who hijacked airliners and flew them into buildings in New York and Washington; and others.
His trial is due to start in October 2002.
There has been much speculation about a 20th hijacker who should have been on board United Airlines flight 93 that crashed into the Pennsylvania countryside - the only flight to have four rather than five hijackers on board. At first, investigators believed it may have been Zacarias Moussaoui.
Focus has now shifted to Ramzi Binalshibh, 29 , also known as Ramzi Omar, a Yemeni who shared an apartment in Hamburg with the suspected ringleader Mohamed Atta. Mr Binalshibh tried and failed to enter the US three times over the summer, according to newspaper reports. He had wired a downpayment of $2,200 to attend a flight school in Florida alongside another of the named hijackers, Ziad Jarrah, but could not obtain a visa - apparently because of a suspected link to the bombing of the USS Cole in Aden harbour in October last year.
He and two other suspects sought by German police, Said Bahaji and Zakariyah Essaabar, are believed to have left Hamburg shortly before 11 September.
A Muslim cleric, Abu Qatada, has been named by UK authorities as being a suspected al-Qaeda fixer based in London, and his assets have been frozen. A judge in Spain goes even further, calling him al-Qaeda's spiritual leader in Europe.
He has been sentenced in his absence to life imprisonment in Jordan for a series of alleged terrorist offences - which he denies - including a plot to kill American tourists around the time of the millennium. Some say he met Osama Bin Laden in Peshawar in 1989, although he denies ever meeting him, or having any connections with al-Qaeda.
Several men with alleged ties with al-Qaeda have said in court testimonies that Abu Qatada's teachings have attracted a mass following among radical Muslims in Europe. Videotaped sermons by Abu Qatada were found in a Hamburg apartmentthat had been used by three of the men alleged to have hijacked the planes that crashed into the World Trade Center, according to the Washington Post.
Qatada - whose real name is Omar Uthman Abu Omar - was born in Bethlehem in 1960 and lived in Jordan until 1989 when he fled the country, alleging political persecution. He arrived in the UK from Pakistan in 1993.
One of the men arrested in connection with an attempt to blow up the US embassy in Paris, Djamel Beghal, said he became interetsted in radical Islam because of Qatada.
The Americans describe him as a "key terrorist recruiter and operational planner and member of Osama Bin Laden's inner circle".
He is alleged to have been heading the reorganisation of al-Qaeda in Pakistan as well as planning new attacks when he was arrested in Pakistan in March.
The 30-year-old, who is believed to have been born to Palestinian parents in Saudi Arabia, is also known as Zayn al-Abidin Mohammed Husain and Abd al-Hadi al-Wahab but has used dozens of other aliases.
He has strong connections with Jordanian and Palestinian groups and was sentenced to death in his absence by a Jordanian court for his role in a thwarted plot to bomb hotels there during millennium celebrations.
US officials believe he planned the "millennium plot" at Los Angeles airport, is connected to a plan to blow up the US embassy in Sarajevo, and a plot to attack the US embassy in Paris. He is also thought to have briefed Richard Reid - the so-called shoe bomber - who was arrested on board a flight from Paris to Miami in December 2001.
Since his arrest he has apparently warned investigators of other plans to attack US interests.
The US authorities believe the 37-year-old Kuwaiti is a leading figure in the al-Qaeda network and helped to plan the 11 September attacks.
They accuse him of working with Ramzi Ahmed Yousef who co-ordinated the first bombing of the World Trade Center in 1993. Yousef was convicted in 1998 and sentenced to 240 years in jail for the original attack which killed six people.
He has also been indicted in the US on charges that he was involved in an unsuccessful attempt to blow up American airliners over the Pacific - the so-called Manila plot.
Zammar, a German citizen, was arrested in Morocco after he left Germany in the wake of the attacks.
It is believed that the Moroccans sent him to Syria.
Zammar is believed to have been in Hamburg with Atta and other members of Atta's cell - including hijackers Marwan al-Shehhi and Ziad Jarrah.
German authorities have said they interviewed him after the 11 September attacks.