A supergrass with al-Qaeda connections has told a UK trial he decided to fight the US despite his mother nearly being killed in the 11 September attacks.
Mohammed Babar is expected to be a key witness
Mohammed Babar, 31, gave evidence in the Old Bailey trial of seven men who deny plotting a bombing campaign.
He revealed that his mother had worked in the World Trade Center building and the attack made him more politicised.
Babar has pleaded guilty in the US to terror offences, but has been given immunity from prosecution in the UK.
Four men, of Crawley, West Sussex, one of Horley, Surrey, one of Ilford, east London, and one from Luton deny conspiring to cause explosions.
Four of the men also deny having chemicals suitable for bomb-making. The trial is expected to last five months.
Babar claims he trained with the men and was influenced by radical preachers like Abu Hamza al-Masri and Omar Bakri Mohammed.
The men, one of whom cannot be pictured, deny all the charges
When asked in court about the 11 September attack on the World Trade Center, Babar said his mother had managed to escape.
"My mother worked in the World Trade Center when the first World Trade Center was hit. She was in the building but she survived," he said.
Babar arrived at the Old Bailey under police escort from a high-security police station in north London. Armed police stood guard at the courthouse door while he gave his testimony.
He told the court he became increasingly politicised and had been seeking unity for the Muslim world since the early 1990s.
Babar, a Pakistani-born US citizen, told the court he had been in contact with Abu Hamza after the 11 September attacks and had been a follower of Mr Mohammed since the early 1990s.
Babar had been studying pharmacy at St John's University in Queens, New York, but dropped out after a year and undertook various jobs, such as valet parking.
His religious views hardened following the first Gulf War and he progressed through various groups including Mr Mohammed's al-Muhajiroun.
"Most influences started in the early 1990s - Sheikh Omar Bakri Mohammed here in the UK," he said.
"They (al-Muhajiroun) had representatives in New York. I was able to meet them on the internet.
"We spoke numerous times over the phone and there was also a lot of literature available on the internet I was able to see."
Babar said Abu Hamza was also an influence via the internet and he later made contact with Abu Hamza.
Babar also told the court he had wanted to fight a Jihad in Chechnya and the Palestinian territory but had not been able to make the right contacts.
He said he knew that Afghanistan was his only chance.
When asked by David Waters QC if he knew he would be fighting against the US, he replied "yes".
Security was particularly tight at the Old Bailey
Babar told the Old Bailey he initially travelled to the UK and then to Pakistan, with the intention of going to Afghanistan.
In Pakistan he met a number of Britons mainly from the London and Crawley areas, he told the court.
"Those brothers who came basically from England and were in Pakistan at the time after 9/11, 15 or 20 of us came to Pakistan for the Jihad.
"The majority had Pakistani ancestry and were from the UK, basically from Crawley and London."
He added that the term brothers related to Muslim brothers and could be used to mean Arabs or members of al-Qaeda.
On Wednesday, the Old Bailey heard the seven accused had targeted Kent's Bluewater shopping centre and a central London nightclub.
One suspect was allegedly involved in a plot to buy a "radio-isotope bomb".
Suspects Salahuddin Amin, 31, from Luton, and Omar Khyam, from Crawley, were alleged by the prosecution to have received training in explosives and use of the poison ricin in Pakistan.
Mr Amin, 31, Jawad Akbar, 22, Mr Khyam, 24, his brother Shujah Mahmood, 19, and Waheed Mahmood, 34, all of Crawley, West Sussex, Anthony Garcia - also known as Rahman Adam - 23, of Ilford, east London, and Nabeel Hussain, 20, of Horley, Surrey, deny conspiring to cause explosions.
Mr Khyam, Mr Garcia and Mr Hussain deny possessing ammonium nitrate fertiliser.
Mr Khyam and Shujah Mahmood deny possessing aluminium powder.