ALEXANDRIA, Va.—Zacarias Moussaoui was a clown who could not keep his mouth shut, according to his old al-Qaida boss, Khalid Shaikh Mohammed. But Moussaoui was surprisingly tame when tried for the 9/11 attacks—never turning the courtroom into the circus of anti-U.S. tirades that some fear Mohammed will create at his trial in New York.

And that wasn’t the only surprise during Moussaoui’s six-week 2006 sentencing trial here — a proceeding that might foreshadow how the upcoming 9/11 trial in New York will go.

Skeptics who feared prosecutors would be hamstrung by how much evidence was secret were stunned at the enormous amount of classified data that was scrubbed, under pressure from the judge, into a public version acceptable to both sides.

Prosecutors were surprised when they failed to get the death penalty — by the vote of one juror.

No one was more surprised than Moussaoui himself: At the end he concluded an al-Qaida member like him could get a fair trial in a U.S. court.