As the first day of jury selection ended in the Colorado cinema shooting case, it was a very different James Holmes at the defence table.

The jail uniform he wore at his initial hearing was replaced with khakis, an untucked blue shirt with white stripes and a blue blazer. His hair, now a dark brown, was neatly trimmed.

The former graduate student whose lawyers acknowledge he opened fire at a midnight Batman film in 2012 also had a curly, medium-length beard and wore oval-shaped reddish glasses.

No restraints were visible, though the judge had ordered him to be tethered to the floor in a way the public could not see for the trial.

Holmes' more conventional appearance was an indication the case was drawing closer to the time when a jury would see the defendant accused of killing 12 people and wounding 70 others at a suburban Denver cinema. But first lawyers have to sort through thousands of potential jurors.

Court officials in Centennial, Colorado, initially summoned a jury pool of 9,000 people, the largest in the nation's history.

But that figure later fell to 7,000 after some summons could not be delivered and some people were excused. The pool will be reduced to a handful in the weeks ahead.

It could take until June to seat the jurors and alternates for a trial that might last until October.

Holmes, who has pleaded not guilty by reason of insanity to murder and attempted murder charges, could get the death penalty if convicted.

The first wave of jurors arrived in court after undergoing two security screenings. They heard instructions from the judge before they would fill out questionnaires.

Judge Carlos Samour suggested earlier that lawyers might not have to screen all the prospective jurors before beginning to select panellists.

He said the process could stop after a few thousand people are screened if both sides agree they have a large enough pool of people.

The scope of jury selection and the trial are testaments to the logistical hurdles of trying the rare case of a mass shooter who survives his attack.

The case has sparked an emotionally charged debate, with Holmes' parents begging for a plea deal that would save his life, while many survivors and family members of victims have demanded he be executed.

After the July 20, 2012, shooting, Holmes, 27, was arrested as he stripped off combat gear in the car park of the Century 16 cinema in Aurora.

If jurors convict him, they must then decide whether to recommend the death penalty. If Holmes is acquitted, he would be committed to the state mental hospital indefinitely.

Defence lawyers accept Holmes was the gunman in the attack but say he was in the grip of a psychotic episode at the time.

Under Colorado law, defendants are not legally liable for their acts if their minds are so "diseased" that they cannot distinguish right from wrong.

Part of the reason the case has dragged on is the battle over whether that standard applies to Holmes.

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