Convicted al-Qaeda plotter Zacarias Moussaoui has told a US court he lied to US officials to stop them uncovering the 11 September 2001 terror plot.
Moussaoui has said "I am al-Qaeda"
He denied direct involvement in the New York attacks, but said he was training to attack the White House in a fifth hijacked plane on 11 September.
Moussaoui said he was to be accompanied by British shoe bomber Richard Reid.
Prosecutors are seeking the death penalty for Moussaoui, who has pleaded guilty to six counts of conspiracy.
Earlier, court-appointed defence lawyers tried to stop Moussaoui giving evidence in an effort to stop him incriminating himself on the stand.
The Moroccan-born French citizen pleaded guilty last year to conspiring with al-Qaeda to attack the US. He is the only man to be charged in connection with the 2001 attacks.
British-born Richard Reid was caught after an abortive attempt to blow up an American Airlines plane heading from Paris to Miami in December 2001.
He was sentenced to life in January 2003.
Under cross-examination by prosecutor Robert Spencer, Moussaoui told the court he had lied after being arrested in Minnesota in August 2001.
Although he did not know exactly when the attacks planned against New York and Washington were due to take place, Moussaoui said he realised that misleading investigators would ensure they were carried out.
"I had knowledge that the two towers would be hit but I didn't have the detail," he told the court.
Zacarias Moussaoui said he knew the attack would kill Americans
Instead he described how he was training as part of the plot that saw planes smash into the World Trade Center and the Pentagon on 11 September 2001.
Moussaoui said he and Reid were due to hijack a fifth plane and fly it into the White House.
He contradicted testimony given when pleading guilty last year, when he said the White House attack was not part of the main 11 September plot.
When asked who ordered him to do this, Moussaoui replied: "Osama Bin Laden."
He told the court that his involvement with al-Qaeda had been "gradual", and that he initially turned down invitations to join the hijacking gang.
Moussaoui was arrested on 16 August 2001, three weeks before the eventual attacks, as he attempted to speed his way through a pilot's course.
US prosecutors have been aiming to prove that Moussaoui deceived federal agents who could have prevented the attacks with more information.
Moussaoui has become well-known for his unpredictable outbursts while in the witness box.
During frequent appearances before the US courts he has sometimes contradicted earlier testimony and appeared mentally unstable.
However, the BBC's Justin Webb, in Washington, says it looks like the prosecution has received something of a gift from Moussaoui.
They have said all along that he knew enough about the 11 September plot to stop it happening, and that appears to be what he has said in court, our correspondent adds.