The suspect in the Colorado shooting rampage displayed behaviour that a gun range owner thought was "bizarre", but it is still unclear if anyone at the university where he studied had any hint of his plans.

Police said James Holmes began buying guns at Denver-area stores nearly two months before Friday’s shooting in Aurora.

He had also received at least 50 packages in four months at his home and at the University of Colorado.

Police are investigating these deliveries to see whether they contained materials for the potentially deadly booby traps police found in his apartment.

Holmes is now being held without bond on suspicion of multiple counts of first-degree murder after a shooting rampage minutes into a premiere of the new Batman film, 'The Dark Knight Rises', that left 12 people dead and 58 injured.

When arrested authorities said Holmes had dyed his hair red and called himself "the Joker" in a reference to Batman’s comic-book nemesis.

He was scheduled for an initial court hearing today and had been assigned a public defender.

Holmes was being held in solitary confinement at a Denver-area county detention facility and was not cooperating with authorities, Aurora police chief Dan Oates said.

"He lawyered up. He's not talking to us," he said.

During the attack, the gunman set off gas canisters and used the military-style semiautomatic rifle, a shotgun and a pistol to open fire on the unsuspecting theatre-goers, Oates said.

The semiautomatic assault rifle used is believed to have jammed during the attack, forcing the gunman to switch to another weapon with less firepower, a federal law enforcement official said. That malfunction and weapons switch during the rampage might have saved some lives.

Holmes had bought weapons at local gun stores in the past two months. He recently purchased 6000 rounds of ammunition over the internet, Oates said.

Police have said that a 100-round drum magazine was recovered at the scene and that such a device would be able to fire 50 to 60 rounds a minute.

The owner of a gun range told AP that Holmes applied to join the club last month but never became a member because of his behaviour and a "bizarre" message on his voicemail.

He emailed an application to join the Lead Valley Range in Byers on June 25 in which he said he was not a user of illegal drugs or a convicted felon, said owner Glenn Rotkovich.

When Rotkovich called to invite him to a mandatory orientation the following week, he said he heard a message on Holmes' voicemail that was "bizarre — guttural, freakish at best".

He left two other messages, but eventually told his staff to watch out for Holmes at the July 1 orientation and not to accept him into the club, Rotkovich said.

QUIET FALLS AT UNIVERSITY

Three days after the massacre, it still remained unclear whether Holmes’ professors and other students at his 35-student Ph.D programme noticed anything unusual about his behaviour.

His reasons for quitting the programme in June, just a year into the five-to seven-year programme, also remained a mystery.

Holmes recently took an intense, three-part oral exam that marks the end of the first year. Those who do well continue with their studies and shift to full-time research, while those who don’t do well meet with advisers and discuss their options, including retaking the exam.

University officials would not say if he passed, citing privacy concerns and also gave no reason for his withdrawal, a decision he made in June.

Holmes was not allowed access to the institution after his withdrawal, which was "standard operating procedure" because he was no longer affiliated with the school, University of Colorado medical school spokeswoman Jacque Montgomery said.

At least two dozen professors and other staff declined to speak with the AP. Some said they were instructed not to talk publicly about Holmes in a blanket email sent to university employees.

Montgomery said that police have told the school not to talk about Holmes.

The university also took down the website for its graduate neuroscience programme on Saturday.

This was done at the request of faculty and staff who had privacy concerns, Montgomery said.

The University of Colorado also disclosed it was cooperating with police who were looking into whether Holmes used his position as a graduate student to order materials in the potentially deadly booby traps that police said they found in his apartment.

The apartment was booby trapped with jars of liquids, explosives and chemicals that could have killed "whoever entered it," Aurora Police Chief Dan Oates said, noting it would have likely been one of his officers.

Investigators spent hours removing the explosive materials Saturday.

POTENTIAL LOST

A resume posted on the job search website Monster.com paints Holmes as a brilliant young man brimming with potential.

He worked as a summer intern at the Salk Institute for Biological Studies in La Jolla in 2006 and mapped the neurons of Zebra finches and studied the flight muscles of hummingbirds while an undergraduate at the University of California.

Ritchie Duong, a friend who has known Holmes for more than a decade, told the Los Angeles Times that in high school he liked to play cards and video games.

They both attended undergraduate school at the University of California, where they saw each other once a week to watch the TV show 'Lost'.

Duong last saw Holmes in December when they met for dinner in Los Angeles and saw a movie together. His friend seemed fine, he told the newspaper.

Academics came easily to Holmes both at high school and at the UC, Duong said.

"I had one college class with him, and he didn’t even have to take notes or anything. He would just show up to class, sit there, and around test time he would always get an 'A'," said Duong, 24.

REMEMBRANCE

President Barack Obama headed to Aurora, Colorado, on Sunday to meet families grieving their losses from the rampage that has stunned the nation and rekindled debate about guns and violence in America.

Churchgoers dabbed at their eyes as special prayers were offered in Aurora for those who died in the town of 325,000, which is not far from the scene of the 1999 Columbine High School student massacre.

"Our culture needs to change," Father Mauricio Bermudez told a packed mass at the Queen of Peace Catholic Church.

"What kind of people are we becoming? Today, we must change. Today is the day."

A vigil was scheduled for 6.30pm on Sunday in front of Aurora City Hall organised by civic community and religious leaders.

"We're still reeling," Aurora mayor Steve Hogan said at a memorial late on Saturday for one young shooting victim.

Across the street from the movie theatre, a man who placed 15 crosses near Columbine High School after the 1999 massacre there, has returned to Colorado with 12 crosses for the victims of Friday’s shooting.

Greg Zanis, of Aurora, Illinois, put up the 1m-tall crosses Sunday on a hill across the street from the Century 16 theatre.

The incident also reverberated in Rome, where on Sunday, Pope Benedict expressed dismay and sadness at the shooting.

"I was deeply shocked by the senseless violence which took place in Aurora, Denver," he said in his regular Sunday Angelus address.

"I share the distress of the families and friends of the victims and the injured, especially the children," he said.

"DIABOLIC, DEMONIC"

Colorado Governor John Hickenlooper, speaking on NBC television’s Meet the Press programme, said there were still no clues as to the motive for the shooting. But he said Holmes was "a twisted, really delusional individual" who had not been cooperating with authorities.

"He was diabolical, demonic," Hickenlooper said.

"I think of him almost as a terrorist. He wanted to take away, not just from the people here but from the country, our ability to enjoy life," he said.

On Saturday afternoon, the local coroner's officer released the names of the 12 people killed, including those of the six-year-old girl, a young man celebrating his 27th birthday and an aspiring sportscaster who had barely escaped a shooting in a Toronto mall earlier this summer.

Those who witnessed the shooting told of a nightmarish scene, with dazed victims bleeding from bullet wounds, spitting up blood and crying for help.

OBAMA WEEPS

US President Barack Obama has wept with victims of the horrific Colorado cinema shooting and told people in the stricken town of Aurora that brighter days are ahead.

"I come to them not so much as president as I do as a father and as a husband," Obama said, visibly holding back the tears even as he addressed reporters after consoling the survivors and the relatives of those killed.

The president described the shooting incident, which left 12 people dead and 58 injured at a local movie theatre on Friday, as an "evil act". But he said the attention surrounding the shooter "will fade away and in the end, after he has felt the full force of our justice system, what will be remembered are the good people who were impacted by this tragedy".

Despite welling up, the president fought back his tears and tried to give an upbeat message in a press conference televised live across America. "You see young people who have come in and just two days ago or 36 hours ago or even 24 hours ago, it wasn't certain whether they would make it and now suddenly their eyes are open, they are alert, and they are talking and it reminds you that even in the darkest of days ... life continues," he said.

Obama ended his remarks by recounting one particular tale of heroism he was told by 19-year-old Allie Young and her friend Stephanie Davies. "When the gunman initially came in and threw the canisters, he threw them only a few feet away from Allie and Stephanie who were sitting there watching the film," he said.

"Allie stood up seeing that she might need to do something or at least warn the other people who were there and she was shot in the neck and it punctured a vein and immediately she started squirting blood." Obama said.

"Apparently as she dropped down on the floor, Stephanie, 21-years-old, had the presence of mind to drop down on the ground with her, pull her out of the aisle, place her fingers over where Allie had been wounded and applied pressure the entire time while the gunman was still shooting."

Obama said that although Allie told Stephanie she needed to run, Stephanie refused to go and instead called 911 with her one remaining hand on her mobile phone.

Once the SWAT team arrived and the shooter had been apprehended, Stephanie then helped others to carry Allie across two parking lots to the ambulance.

"And because of Stephanie's timely actions, I just had a conversation with Allie downstairs and she is going to be fine," Obama said. "I don't know how many people at any age would have the presence of mind that Stephanie did or the courage that Allie showed. And so as tragic the circumstances of what we've seen today are, as heartbreaking as it is for the families, it's worth us spending most of our time reflecting on young Americans like Allie and Stephanie," he said. "Because they represent what's best in us and they assure us that out of this darkness, a brighter day is going to come."

-AAP

-AP

- Reuters