Normally, evidence about a defendant's life and mental health doesn't get introduced until sentencing.

Question of timing

But legal experts said it was essential that the defense make that case early in the trial, before the prosecution had a chance to put on the emotional and powerful victim impact testimony that could come in the sentencing phase of the trial.

"Without insanity, none of this would have been heard until after a capital prosecution and after the prosecution's case for death," Benjamin said.

"You want to talk about your strongest point as soon as you can. Otherwise you're buried."

The defense's case for insanity was far from perfect.

Among the biggest holes was the admission by a defense team psychiatrist that Malvo told her he felt troubled by the shooting of a 13-year-old boy at a middle school in Bowie.

Diane Schetky, a psychiatrist from Rockport, Maine, said Malvo hadn't realized that shooting children was part of Muhammad's extortion plan.

"He indicated he was conflicted about shooting the child, which is why it wasn't a head shot," Schetky testified. "He didn't practice shooting children. He practiced shooting adults. They didn't have cutouts of children on the trees where they practiced."

Malvo's letters

Jurors also heard of a letter Malvo wrote two months before the Washington-area sniper shootings in which he expressed fear that Muhammad would kill him and called himself a ticking time bomb close to exploding.

While the letter might show the control that Muhammad exerted, it can also be seen as showing Malvo was aware of the control.

"I have a father who I know is going to have to kill me for a righteous society to prevail," said the letter, which was written to a niece of Muhammad's in Baton Rouge, La.

Jurors also were told of letters Malvo wrote to a fellow inmate at the Fairfax County Detention Center in the late summer and fall of this year. By that time, Malvo's psychiatrists claimed he had broken free of Muhammad's spell.

But experts say the letters could lead a jury to believe Malvo is just as manipulative and cunning as Muhammad.

Malvo writes in one letter, "I play the stupid fool. Look at how I act and speak. Everybody underestimates me. ... I love that, it gives me the edge I need to study, conquer and overcome."

Staff writer Andrea F. Siegel contributed to this article.