As more details of the Colorado shooting tragedy that killed 12 and wounded 58 were released over the weekend, arguments on gun control heated up in the United States, a country known for its gun culture.

Though US President Barack Obama was scheduled to travel to Colorado on Sunday to visit the families of victims, few expected the discussion about guns to yield tangible progress, due to the strength of gun-rights advocates.

Holmes, the suspect, had planned the movie theater massacre in the Denver suburb of Aurora, authorities said.

The dead included a 6-year-old girl, two members of the US military, a woman who escaped a mall shooting in Toronto last month, a man celebrating his 27th birthday and two men who sacrificed their lives to save the women they were with.

All but one were under the age of 32, and eight were in their 20s.

Police said Holmes planned the attack with "calculation and deliberation", receiving multiple deliveries of ammunition by mail for months before the attack.

The suspect bought at least 6,000 rounds of ammunition, an AR 15 assault rifle, a Remington shotgun and two 40-caliber Glock handguns.

His apartment was rigged with jars of liquids, explosives and chemicals that were booby-trapped to kill "whoever entered it", Aurora Police Chief Dan Oates said.

All hazards have been removed from the apartment.

Police took Holmes into custody on Friday in a parking lot behind the cinema. He is expected to make his first court appearance on Monday.

All of the major studios in Hollywood said on Saturday they were joining Dark Knight Rises distributor Warner Bros in withholding their box office numbers for the weekend, out of respect for the victims and their families.

Warner Bros also rushed to remove a movie trailer with now-eerie relevance. The trailer of the movie Gangster Squad, which stars Sean Penn and Ryan Gosling, features a climactic scene in which mobsters fire automatic weapons into a movie theater audience from behind the screen. Obama, who called in his weekly radio address for prayer and reflection following the rampage, was scheduled to travel to Colorado on Sunday to visit the families of victims.

A vigil was scheduled for 6:30 pm on Sunday in front of Aurora City Hall.

Republican presidential challenger Mitt Romney also scaled back his campaign schedule in the wake of the massacre and called for unity.

However, in their initial comments on the Aurora shootings, neither Obama nor Romney mentioned gun control, which is considered politically toxic in an election year.

"If there's anything to take away from this tragedy, it's a reminder that life is fragile. Our time here is limited and it is precious," the president said in his weekend address.

Major US shooting tragedies in the past usually sparked fierce debate on gun control, yet ended up with nothing conclusive.

Speaking on CNN on Friday, Harvard University professor Laurence Tribe blamed the impasse on powerful gun rights defender groups, mentioning "the National Rifle Association and all of the people who, frankly, make a living out of restricting the political possibility of gun control".

Tribe said: "We have to do something about it. I don't know how many killings, how many slaughters it's going to take before the nation wakes up to the need to address the problem."

According to the website of the Brady Campaign to Prevent Gun Violence, named after James Brady, who was shot and permanently disabled during the 1981 attempt to assassinate former US president Ronald Reagan, there are an estimated 283 million guns in civilian hands in the US, almost one gun for every man, woman and child in the country.

Current federal law requires criminal background checks only for guns sold by licensed dealers, which account for just 60 percent of gun sales, meaning that two out of every five guns acquired in the US are sold without a background check.

This includes guns bought at gun shows, between individuals and even through the Internet.

Meanwhile, about 100,000 people are shot every year, and more than 30,000 of them die.

Since Martin Luther King and Robert Kennedy were gunned down in 1968, about 1 million Americans have died because of gun violence.

Colorado law does not force residents to register guns and follows a "shall issue" statute for carrying concealed weapons, which means that local law enforcement is required to provide licenses to citizens such as Holmes who want to carry concealed weapons as long as they meet certain criteria. The authorities have only limited data on active gun owners.

The New Republic's Timothy Noah blogged on Saturday morning about that tear gas grenades "are perfectly legal in Colorado, even though there is no legitimate need - none - for someone not in law enforcement or the military".

Holmes was reported to have thrown tear gas grenades and smoke shells into the audience on Friday.

Contact the writers at lixiaokun@chinadaily.com.cn and chenjia@chinadailyusa.com.

Qin Zhongwei, Reuters and AP contributed to this story.