Zacarias Moussaoui said he rejoiced as the Twin Towers burned

Zacarias Moussaoui has often been referred to as "the 20th hijacker" but at the time of the 11 September 2001 attacks he had already been arrested in the US.

A French citizen of Moroccan descent, the 37-year-old has spent almost five years in custody and is the only person charged in the US over the attacks.

It is thought he began a journey into Islamic radicalism in Britain - studying at London's South Bank University and worshipping at the Finsbury Park mosque.

But whether the self-confessed al-Qaeda operative was a serious plotter or just a fringe figure has never become clear.

Is there, somewhere deep down in there, still a core of the old Zacary people knew and loved underneath all the extremism? Moussaoui's mother Aicha

In court at his sentencing trial, Moussaoui said he had been part of a grandiose plot to fly a Boeing 747 into the White House.

Yet testimony from 9/11 mastermind Khalid Sheikh Mohammed - who has been in US custody for three years - was used by Moussaoui's defence lawyers to undercut his claims.

In testimony written up by US intelligence operatives, Sheikh Mohammed's insisted Moussaoui was never intended to be part of the 9/11 plot.

Rather, he was to be part of a second wave of attacks, using operatives with Western passports thought less likely to draw scrutiny from US authorities.

What is certain is that Moussaoui was detained three weeks before 9/11 on immigration charges after a flying school in Minnesota reported that he had been acting suspiciously.

He had been on a jumbo jet simulator but had shown no interest in take-offs or landings - only in how to control the plane in the air.

Jurors concluded that his silence in the run-up to 9/11 made him responsible for at least one death on that day, and made him eligible for the death penalty.

Shoe bomber link

Moussaoui was born in May 1968 in south-west France and came to Britain in the early 1990s to enrol as a student.

He graduated in 1997 with an MA in international business studies.

Moussaoui is thought to have become an extremist in the UK

He lived in south London on and off for nine years.

His brother, Abd-Samad Moussaoui, was quoted as saying that Zacarias became involved in Islamic extremism while living in London.

French Intelligence was reportedly interested in him and warned the Americans, but Moussaoui was always one jump ahead and nobody "joined up the dots" about him, even after he was detained.

He attended the Finsbury Park mosque, at a time when it was linked to extremist activities. Convicted British shoe bomber Richard Reid also attended the mosque.

Moussaoui later said in court that Reid was set to join him on his mission to crash a plane into the White House - a claim some saw as part of a bid by Moussaoui to secure the death penalty and cement a place as an Islamic martyr.

The FBI concluded there was no evidence that Reid had prior knowledge of 9/11, or that al-Qaeda had told him to work with Moussaoui.

Cash payment

According to the indictment against him, Moussaoui went for training at an al-Qaeda camp in Afghanistan in 1998.

Aicha el-Wafi no longer recognises her son

The US commission which investigated the 9/11 attacks found that Moussaoui was ordered to undergo flight training in Malaysia in late 2000 by Khalid Sheikh Mohammed, but was unable to find a school he liked.

Moussaoui was in Pakistan at the end of 2000 before going to London and then onto the US in February 2001.

There, he enrolled in one flying school in Oklahoma and then paid $6,000 cash to join the school in Minnesota.

The statement of facts signed by Moussaoui when he pleaded guilty to conspiring with al-Qaeda included a paragraph that said bin Laden had personally approved of him attacking the White House.

'Happy boy' became fanatic

Moussaoui's mother, Aicha, who lives in France but attended her son's sentencing trial in the US state of Virginia, says he is innocent of involvement in the 9/11 attacks and is part of a show trial.

She accuses British extremists of turning him from a "happy boy" into a fanatic.

But she says she stopped making phone calls to him last May, because she no longer recognised her son.

Moussaoui's outbursts and invective-filled letters to officials led to the trial judge revoking his right to represent himself in 2003.

Later he said that in his cell he rejoiced as the Twin Towers burned and collapsed.

His defence argued that Moussaoui was mentally ill. His mother admits she is not sure.

"Is there, somewhere deep down in there, still a core of the old Zacary people knew and loved underneath all the extremism?" she told Time magazine in April 2006.

"Often, during the phone calls, I knew there was. After these outbursts, it was hard to know."