Moussaoui will spend 23 hours a day in a solitary cell for the rest of his life, writes Denis Staunton.
In refusing to sentence Zacarias Moussaoui to death, the jury of nine men and three women concluded the prosecution had failed to persuade them the self-confessed al-Qaeda terrorist was personally responsible for the deaths of 3,000 people on September 11th, 2001.
A death sentence would have required a unanimous verdict but the jury could not agree that Moussaoui caused the 9/11 deaths or that he committed his crimes "in an especially heinous, cruel or depraved manner".
Moussaoui admitted he came to the US as an al-Qaeda fighter to learn to fly so he could crash an aircraft into a crowded building. Prosecutors acknowledged he did not take part in the 9/11 attacks and was in jail that day on an immigration violation. The prosecution argued, however, that if Moussaoui had told investigators all he knew, the attacks could have been prevented.
Defence lawyers portrayed the FBI investigation as so badly bungled that no information Moussaoui could have provided would have prevented the attacks. And they argued he was a marginal, mentally unstable figure not trusted with important knowledge by al-Qaeda.
Moussaoui pleaded guilty to conspiracy charges last year and the jury agreed he could be considered for the death penalty. The jurors' task this week was to assess aggravating factors identified by the prosecution which would justify the death penalty, along with mitigating factors put forward by the defence which would justify sparing his life.
The jury agreed with prosecutors that Moussaoui "knowingly caused a grave risk of death to one or more persons" and committed the offence after "substantial planning and premeditation". They agreed his actions caused physical and emotional injuries to 9/11 victims and harm to their friends and relatives, and caused disruption and destruction to New York.
The jury could not agree, however, that Moussaoui's actions resulted in the deaths of 3,000 people or that he committed his crimes "in an especially heinous, cruel or depraved manner".
The jury unanimously rejected some of the mitigating factors presented by the defence, notably that Moussaoui was schizophrenic and actually wanted a "martyr's death". Nine out of 12 jurors agreed, however, that Moussaoui's unstable childhood - partly spent in an orphanage - and his father's violent temper should be taken into account.
Three agreed the racism he suffered as a Moroccan child in France had affected him deeply, and three noted he was unlikely to present a risk if he was imprisoned for life. Three jurors wrote in a mitigating factor not identified by the defence - Moussaoui had limited knowledge of 9/11 plans.
Moussaoui's lawyer, Edward MacMahon, described his client as "impossible" and said he would not be appealing the life sentence. Moussaoui refused to co-operate with his defence team, accusing the lawyers of wanting to kill him.
Moussaoui's life sentence offers no chance of parole. He will spend the rest of his life in a maximum security prison, spending 23 hours each day in solitary confinement with just an hour of exercise. He will be kept in a "supermax" Administrative Maximum prison in Florence, near Denver, Colorado.
Cells feature a bed, desk, stool and shower, with a small black-and-white television broadcasting anger-management lessons, literacy classes and religious services. Inmates, who include Ramzi Yousef, who planned the 1993 World Trade Center bombing, Unabomber Theodore Kaczynski and Oklahoma bomber Terry Nichols, have their wrists cuffed behind their backs each time they leave their cells.
