Ramzi Binalshibh was regarded as one of the most senior al-Qaeda members when captured in Pakistan in September 2002.
Binalshibh was said to be part of an al-Qaeda cell in Hamburg
Mr Binalshibh - a Yemeni national - was named among the most wanted suspects within weeks of the 11 September 2001 attacks on the United States and is now set to face charges brought by the Pentagon over the attacks.
Mr Binalshibh was held at an undisclosed location by the CIA until September 2006, when he became one of a number of prisoners who US President George W Bush said had been transferred to Guantanamo Bay in Cuba.
Mr Binalshibh, who is in his 30s, 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.
Moussaoui's indictment mentioned Binalshibh
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.
Mr Binalshibh is the only person believed to have attended both of the crucial meetings held to plan the operation, one in Malaysia and the other in Spain.
He handled logistics and money matters for the attacks and entered Pakistan just before 11 September, US officials say.
Mr Binalshibh 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.
His place was reportedly taken by Zacarias Moussaoui - the only person to have been charged - and then jailed for life - in the United States in connection with the attacks.
Mr Moussaoui was allegedly sent money by Mr Binalshibh.
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 in 2002, in which 14 German tourists were killed.
In his combatant status review at Guantanamo Bay, Mr Binalshibh was described as "uncooperative and unresponsive", vowing to have nothing to do with the process.
A pre-trial hearing was held in January 2009 to determine whether Mr Binalshibh was mentally competent to represent himself.
Amid chaotic scenes, he told the court in Arabic that he was "proud" of the 9/11 attacks.