Batman gunman trial begins

The trial of James Holmes, who is charged with killing 12 moviegoers in a shooting spree in a cinema in Aurora has started with jury selection. Photo / Brennan Linsley AP

The gunman accused of killing 12 US cinema-goers at a 2012 screening of a Batman movie has appeared in court, as his long-awaited trial finally got under way with jury selection.

James Holmes sported a neatly trimmed beard, dark brown hair and dark-rimmed glasses as he sat listening to Tuesday's proceedings in Centennial, Colorado.

The 27-year-old has pleaded not guilty by reason of insanity, but could face the death penalty if convicted.

Holmes has been in custody since the night of the mass murder on July 20, 2012 in Aurora and faces 166 counts of aggravated murder, attempted murder and possession of explosives.

The trial began with jury selection - likely to last a few months - to pick 24 jurors and stand-ins from a selection pool of over 9,000 Denver-area residents, whittled down to 7,000 by Tuesday.

Holmes smiled and chatted with his lawyers. He wore no handcuffs during the morning hearing, but did have his legs shackled.

Witnesses say Holmes threw smoke bomb-type devices before opening fire inside the Century cinema with weapons including a military-style rifle, a shotgun and a .40-calibre pistol. Seventy people were also wounded.

His one-bedroom apartment was later found to be booby-trapped with an array of homemade explosive devices which police had to disarm before entering.

In preliminary hearings prosecutors said Holmes, a neuroscience graduate student, had in his possession sufficient ammunition to kill everyone in the crowded cinema showing The Dark Knight Rises.

The primary issue at trial will be whether Holmes was sane at the time of the massacre. He has undergone two separate psychiatric examinations since his arrest and much of the trial is expected to be devoted to psychiatric testimony.