Then, yesterday, the French national of Moroccan origin told a hushed courtroom of the plot.
Moussaoui said he was not meant to be part of the September 11 attacks on New York and Washington.
But he said he was part of a broader conspiracy to use hijacked airplanes as "a weapon of mass destruction." Al-Qaeda leader Osama bin Laden had handpicked him, he said, to fly a commercial airliner into the White House.
Moussaoui pleaded guilty to all six charges, including conspiracy to commit acts of terrorism.
Prosecutors say they will seek to put him to death. Attorney General Alberto Gonzales said, "Four of these charges authorize a maximum penalty of death, and as you know, we are seeking the death penalty in this case."
Moussaoui's guilty pleas come after a three-year legal saga.
It began with his arrest on immigration charges in August 2001.
Police picked him up after a flight trainer got suspicious when Moussaoui said he wanted to learn to fly a Boeing 747.
So, he was in custody on September 11 when hijackers flew airliners into the World Trade towers and the Pentagon.
Attending the hearing were several relatives of victims of those attacks.
Among them was Debra Burlingame, whose brother Charles was the captain of the plane that crashed into the Pentagon:
"I think this is a great day for American justice. We have here an iron-clad, an iron-clad guilty plea on six counts of conspiracy in furtherance of the September 11th plot which killed my brother and 3000 other Americans. This is a great day for truth and justice," said Burlingame.
Abraham Scott's wife Janice died in the Pentagon. He said, "Me personally, the death penalty. I'm a Christian and I am willing to forgive, but in this case I can't. The death penalty, I will accept nothing less."
It's now up to the judge to set a date for a sentence hearing. Then a jury will decide if Moussaoui should be put to death.
Moussaoui says he'll fight the death penalty. But he says he doesn't expect "any leniency."