James Holmes, who is believed to have killed 12 people attending a midnight screening of the latest Batman movie in the US town of Aurora, will make his first appearance in court on Monday.

The deaths in the attack, which also left 58 people injured, many of them seriously, have stunned the Denver suburb and shocked Americans.

Monday  will be the first chance for the public to get a look at Holmes, 24, since he surrendered to police in the car park of the Century 16 cinema in the early hours of Friday morning.

He had apparently died his hair red, and referred to himself as the classic Batman villain “the Joker” as he was being arrested. He is alleged to have burst into the cinema wearing all-black bullet-proof clothing, a gas mask and hurling gas canisters and methodically shooting into the packed audience.

Late on Saturday, police defused a string of booby-traps and home-made bombs that Holmes apparently left in his flat.

The court appearance is likely to be brief, as prosecutors must decide whether to seek the death penalty and defence lawyers consider whether they believe Holmes is mentally competent to stand trial. The process is likely to take many months.

More details emerged of the intensive preparations Holmes, a bright and talented science student, carried out in the months before the attack. He apparently spent more than $15,000 (£9,500) on the internet amassing 6,000 rounds of ammunition and his bullet-proof clothing, which covered him from head to toe.

In total, at least 90 packages were delivered to Holmes over a period of four months. Police now have CCTV footage of Holmes collecting his purchases – vital clues for prosecutors seeking to show he carefully planned the crime.

Aurora’s police chief, Daniel Oates, said: “What we’re seeing here is evidence of, I think, some calculation and deliberation.” Holmes bought his four guns in person at local gun shops. All of the guns, ammunition and equipment were bought legally, taking advantage of relaxed US gun ownership laws.

The University of Colorado Denver is investigating whether Holmes used his position as a postgraduate student to obtain dangerous materials.

However, despite his collection of weapons and bullets, it is clear that no one saw Holmes as a serious threat. Though he was struggling to find work, and was leaving his postgraduate studies in Colorado, Holmes has been consistently described as a quiet, shy individual who seemed largely normal and certainly not a potential mass killer.

In a résumé posted on the job-hunting website Monster.com, Holmes listed himself as an “aspiring scientist” and said he was looking to work as a laboratory technician. Yet Holmes, instead of finding work, appears to have dedicated himself to creating spectacular carnage.

He transformed his flat into a lethal trap by priming explosives and turning on music at a high volume. Neighbours fortunately did not open the door at the noise and instead called police. “The apartment was designed to kill whoever entered it,” said Oates, outlining a belief that it might have been aimed specifically at killing Aurora police officers after the shooting.

“If you think we are angry, we sure as hell are angry,” Oates said.

If Aurora’s citizens were lucky that Holmes’ flat did not explode as intended, then they also appear to have been fortunate that one of Holmes’ weapons appeared to have jammed during the shooting at the cinema.

The Associated Press news agency, quoting an unnamed federal law-enforcement official, reported that Holmes’ semi-automatic rifle stopped working, forcing him to change weapons to a gun with a much lower rate of fire. That almost certainly reduced the death toll of the tragedy.

However, that news is likely to be little comfort for relatives of the 12 dead and many wounded. As the identities of all the fatalities emerged at the weekend, it was revealed that the youngest was six-year old Veronica Moser-Sullivan, who had just learned to swim over the summer. Her mother, Ashley Moser, remains in hospital with several gunshot wounds, as yet unaware of her daughter’s death.

Other victims included a local bartender, a serving soldier and restaurant workers. The list of dead and injured was simply a random slice of suburban American life. The only thing that truly united them was the fateful choice to go and see The Dark Knight Rises last Friday.