CENTENNIAL, Colo. -- A jury was seated Tuesday in the death penalty trial of Colorado theatre shooter James Holmes after a selection process that experts say was among the largest and most complicated in U.S. history.

The 12 jurors and 12 alternates were chosen after remaining candidates were questioned as a large group Monday.

About 9,000 prospective jurors initially were summoned in what experts called the nation's biggest-ever jury pool. They spent weeks filling out lengthy written questionnaires.

Hundreds were then asked to return for one-on-one questioning, where defence attorneys, prosecutors and the judge questioned them, sometimes for hours, about their views on the death penalty, mental illness and other aspects of the criminal justice system.

Opening statements are scheduled for April 27.

Holmes is charged with killing 12 people and wounding 70 others in the July 20, 2012, attack on a Denver-area movie theatre. His defence attorneys don't dispute that he pulled the trigger but say he was in the grips of a psychotic episode when he slipped into the theatre and opened fire while dressed from head to toe in combat gear.

Prosecutors insist Holmes was sane and will ask jurors to convict him and sentence him to death.

If the jury finds Holmes was legally insane at the time of the attack, he would be committed indefinitely to the state psychiatric hospital. If the jury convicts Holmes, the only other option other than a death sentence is life in prison.

Jury selection began Jan. 20.

Many potential jurors were excused when they said they already had an opinion on Holmes' guilt or were morally opposed to the death penalty.

Still others were dismissed because of personal connections to the shooting, including people who had friends or family in the packed theatre that night, or who knew some of the hundreds of first responders who rushed to the scene.

In group questioning, attorneys had the chance to dismiss potential jurors without giving cause.