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factors that can bar a gun purchase in the state. Congress also passed a law to give states funding to put more of this kind of data into the FBI Instacheck system that screens buyers in gun stores at the time of purchase.

But even after the Virginia shootings, Congress did not close the national loophole that critics say makes it too easy for people to buy guns privately, or at gun shows, without background checks.

Aside from the expanded mental-health "flags," Wilson said, Virginia Tech had minimal legislative impact. In Colorado, too, state lawmakers have been reluctant to impose tougher gun curbs -- even after the 1999 shootings at Columbine High School in which two teenagers killed 12 fellow students as well as themselves.

Of four guns used at Columbine, three were bought at gun shows. Even so, legislators did not enact background checks at such shows. Ultimately, it took a grassroots movement in the state to push through a constitutional amendment making such checks mandatory.

It now appears that the suspect in Friday's shootings, James Holmes, 24, bought all his guns at national chains -- legally.

CBS News reported that he bought his four guns from three shops between May 22 and July 6. The local police chief said Holmes also bought 6,000 rounds of ammunition on the Internet -- legally

Aurora Police Chief Dan Oates said they also recovered at the shooting scene a 100-round magazine for the rifle.

Such a high-capacity magazine was illegal under the assault-rifle ban, which limited clips to 10 rounds of ammunition.

Wilson downplayed the significance of such a ban, pointing out that clips are easy to change. "People sometimes get hung up on the size of the clip. With a semiautomatic you can squeeze off 10 rounds very, very quickly."

Bloomberg News, quoting an unnamed federal official, said Holmes had no criminal history that would have turned up in a pre-sale background check and barred a gun buy.

In Colorado, according to state officials, the checking system forbids sales to people convicted of crimes punishable by more than a year in prison, and to those with serious mental issues and to anyone convicted of domestic violence, among other rules.

Under federal laws, stores must make a report to the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives when someone returns to a store within five days to make a second gun buy. But the timing of Holmes' buys would not have triggered such reports, Bloomberg said.

Opponents of tougher restrictions remain a force in the Colorado legislature, as they do in many state capitals. Most recently, they have pushed a proposal to abolish Colorado's background check system, arguing that it duplicates federal requirements.

The measure passed the GOP-controlled House this year but has stalled in the Colorado Senate, where Democrats are in the majority.

In Pennsylvania, gun-rights advocates have long been a potent force.

Last year, Gov. Corbett, a Republican, signed a law expanding the so-called castle doctrine to permit people to use deadly force to defend themselves beyond the boundaries of their homes. His Democratic predecessor, Ed Rendell, vetoed a similar measure in 2010, saying, "We shouldn't have a shoot-first mentality."

Pennsylvania State Rep. Daryl Metcalfe (R., Butler), who cosponsored the castle-doctrine bill, views it as a key safeguard for the law-abiding public. "They ultimately are able to protect themselves, their families, and their property," he said.

Some gun-control advocates contend most people don't realize how many loopholes exist in firearms laws.

Daniel Vice, senior attorney with the Brady Center to Prevent Gun Violence, said the president simply hasn't pounded his bully pulpit.

"The lack of White House leadership has made it very difficult to get the message out that we can do something," he said.

But Vice cautioned against assuming that public opinion was frozen beyond any thawing. "People are getting angry." he said. "At every bloody child that is removed from a store or a movie theater people get angry and they want action."

Kristen Rand, legislative director with the Violence Policy Center, in Washington an organization generally regarded as to the left of the Brady Campaign, said random shootings such as the one in Colorado could energize the public -- if only because such episodes have flavor of "it could have been me."

Rand said those opposed to stricter gun control were given too much credit for their electoral clout.

Still, she said, Democrats think this is a political loser. "That's where we are. The problem is, how many people have to pay with their lives for that political perception?"

Representatives of the National Rifle Association, the leading organization opposed to tougher gun-control laws, did not return a telephone call Friday seeking comment. The NRA, which boasts nearly four million members, issued a statement saying:

"Our thoughts and prayers are with the victims, their families, and the community. NRA will not have any further comment until all the facts are known."

On the campaign trail, Obama and Republican challenger Mitt Romney expressed sadness at the deaths but steered well clear of any policy debates on guns.

This left some gun-control advocates fuming. In a radio interview, New York City Mayor Michael Bloomberg said, "Soothing words are nice" but not enough.

"Maybe it's time the two people who want to be president of the United States stand up and tell us what they're going to do about it, because this is obviously a problem across the country," he said.

In Philadelphia, Mayor Nutter also urged a legislative response. The killings "should be an occasion to strengthen gun-safety laws, to call for national reform and to better ensure the safety of the public," Nutter said in a statement.

But Wilson, the professor at Roanoke College in Virginia, said he expects the mayors' call to go unheard.

"In another three months," Wilson said, "this will be another one of those tragic moments in history -- and it won't be a campaign issue."