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 1998 to 1999 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.
Ramzi Binalshibh, a Yemeni national, is is allegedly one of the most senior al-Qaeda members to be arrested over the past year. He was arrested in Pakistan on 11 September 2002.
US Attorney General John Ashcroft named him among the most wanted suspects within weeks of the 11 September attacks on the United States.
Western intelligence officials believe he is the missing link - the one person who can put all the pieces of the al-Qaeda strategy into context.
Mr Binalshibh, who is 30, is said to have become a key member of the al-Qaeda cell in Hamburg, Germany, after seeking asylum there in the late 1990s.
According to officials, he met Mohammed Atta, the leader of the Hamburg cell and one of the alleged masterminds of the 11 September attacks, through a local mosque in 1997.
The two men are said to have become roommates and, over the next two years, to have engaged in radical Islamic activities.
In an interview with the Arab TV station al-Jazeera, Mr Binalshibh said he and other members of the Hamburg cell travelled to Kandahar in Afghanistan in late 1999 to receive training. There, according to the interview, they met many of the key players in the 11 September attacks.
He handled logistics and money matters for the attacks and would have been among the hijackers who carried out the attacks, had he not tried and failed four times to obtain a US visa, according to investigators, US officials say.
His place was reportedly taken by Zacarias Moussaoui - the only person to have been charged in the United States in connection with the attacks.
Intelligence officials say Mr Binalshibh may also have been involved in two other operations blamed on al-Qaeda.
One was the suicide attack in Yemen on the USS Cole, an American destroyer, in which 17 sailors died in 2000.
The other was the attack on a Tunisian synagogue earlier this year, in which 14 German tourists were killed.
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 apartment that 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 Abu Qatada.
Abu Zubaydah is the most senior member of al-Qaeda to be captured so far.
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.
Khalid Sheikh Mohammed
Khalid Sheikh Mohammed is on the FBI's most-wanted list and the reward for his capture has been increased to $25m.
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.
Mr Mohammed is reported, along with Ramzi Binalshibh, to have initially planned to fly hijacked planes into nuclear installations rather than the World Trade Centre. The al-Jazeera Arabic television station reported that it had interviewed the two men who made the claim.
Mohammed Haydar Zammar
US investigators believe that Syrian-born Mohammed Haydar Zammar recruited Mohammed Atta - the suspected ringleader of the 11 September suicide attacks.
Zammar, a German citizen, was arrested in Morocco after he left Germany in the wake of the attacks. Moroccan authorities later 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.
Mounir al-Motassadek, a 28-year-old Moroccan, is currently on trial in Germany where he is accused of supporting the 11 September attacks.
He is charged with membership of a terrorist cell and of being an accessory to the murder of more than 3,000 people.
Mounir al-Motassadek was just one of thousands of young Muslims who arrived in Europe in the mid-1990s to study.
He travelled to Munster in Germany in 1993 where he did a language course. He then joined Hamburg University to study electrical engineering.
During his time there, prosecutors say he met Mohammed Atta– the man they believe piloted the first hijacked aircraft into the World Trade Center.
Prosecutors allege that while in Hamburg, Atta and several others founded a radical Islamic organisation which fostered links with Osama Bin Laden's al-Qaeda network.
Prosecutors say al-Motassadek "maintained intensive contacts over several years" with Atta and other 11 September suspects Marwan al-Shehhi and Ziad Jarrah.
They also believe al-Motassadek acted as the group's "treasurer", handling funds for the living expenses of three of the hijackers - including Atta.
Mr Motassadek first came under suspicion in November 2001 when German authorities discovered that he had power of attorney over a bank account held by al-Shehhi, who allegedly flew the second plane into the World Trade Center.
He was questioned and released, but remained under police surveillance.