And, yes, expanded background checks might have kept Eric Harris and Dylan Klebold from killing 13 people and themselves in the 1999 Columbine massacre. Three of the four guns the two 17-year-olds used in the shootings were purchased for them at a gun show by Robyn Anderson, then 18. "I would not have bought a gun for Eric and Dylan if I had had to give any personal information or submit to any kind of check at all," Anderson said in a statement in 2000. "I wish a law requiring background checks had been in effect at the time."

An expanded background-check system is only as good as its database. New proposed federal laws and President Obama's executive actions are aimed at making sure that mental illness is better detected, reported and treated, and that states have the money to enter mental health adjudications, felony records and domestic violence restraining orders into the system. In fact, the Manchin-Toomey plan would have given states grants to upgrade their databases.

Would this be helpful? Ask the survivors of 32 people killed six years ago at Virginia Tech. Seung-Hui Cho, the shooter, was mentally ill and had been adjudicated as dangerous. But his records weren't entered into the system, so he passed a background check. Virginia fixed its reporting system, but many states still have gaps.

Some pundits, lawmakers, and advocates, topped by the NRA, have argued it would be pointless to limit the size of ammunition magazines. Yet past incidents suggest such limits could make a difference. In 1998, Kip Kinkel emptied a 50-round clip at Thurston High School in Springfield, Oregon, killing two students and injuring 25. When he stopped to reload, several students wrestled him to the ground.

Adam Lanza brought 10 magazines of 30 rounds each into Sandy Hook Elementary School last December. Parents there say 11 children may have escaped when he had to stop to reload. In Tucson in 2011, shooter Jared Loughner was tackled and his gun wrested from him as he tried to reload after firing 31 bullets in a matter of seconds. If his clip had been limited to 10 rounds, Christina Taylor Green might be alive today. She was killed at age 9 by Loughner's 13th bullet.

Brian Malte, national policy director of the Brady Campaign to Prevent Gun Violence, says there's no single way to head off shootings, which is why most gun-control advocates support a wide range of steps. The starting point for all of them is a background check system that covers as many gun purchases as possible. That is how ineligible people can be identified and denied a gun. It's also the foundation for enforcement of state laws like those in California, which limit handgun purchases to one a month (helpful in discouraging trafficking) and require a waiting period before purchase (helpful in preventing suicides).

Newtown parents are aware that a better background check system would not have kept guns away from Lanza, whose mother had a huge cache of weapons and ammunition in their home. But they and others involved in the push for expanded background checks and other new laws are looking toward the future, not the past. "They don't want what happened to them to happen to somebody else," says Malte. "That's the overriding factor."

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