Afterward, President Barack Obama said his administration would do what it can without Congress. And Obama said now that the issue has been revived, it won't go away.
But it's unclear what, if anything, comes next in gun politics. A look at the issue:Twenty children — all first-graders, just 6 or 7 years old — felled by semi-automatic rifle fire within five minutes. Six women — teachers, aides and the principal — gunned down. The shooter also took his own life and, before heading to Sandy Hook Elementary School, killed his mother.
The carnage in Newtown shocked a nation and its leaders. Yet, a shooting takes multiple lives at a high school or college nearly every year. Almost two years before Newtown, a congresswoman was wounded in a deadly attack that led Obama to call for "a new discussion" of gun laws. He didn't press the issue then.
Gun rights supporters say appealing to emotion after such tragedies leads to misguided policies that make it harder for law-abiding Americans to protect themselves. The nation should focus on protecting its schools, the National Rifle Association says.
Among the mass shootings that have most influenced the gun debate:
— July 2012 in Aurora, Colo.: A gunman sprays bullets into a packed theater on the opening night of the Batman movie "The Dark Knight Rises." Twelve people are killed, 70 injured. James Holmes, 25, is awaiting trial, with prosecutors seeking the death penalty.
— January 2011 in Tucson, Ariz.: Six people are fatally shot and 13 injured at a meet-and-greet event outside a supermarket for then-U.S Rep. Gabrielle Giffords. She has become an advocate for stricter gun laws as she recovers from a devastating head wound. Jared L. Loughner, 24, pleaded guilty and is serving life in prison.
— November 2009 at Fort Hood, Texas: Thirteen soldiers and civilians are killed and more than two dozen wounded when a gunman walks into the Soldier Readiness Processing Center and opens fire. Army psychiatrist Maj. Nidal Hasan is awaiting trial.
— April 2007 at Virginia Tech in Blacksburg: In the deadliest U.S. school shooting, student Seung-Hui Cho kills 32 students and faculty in a dorm and a classroom building, then commits suicide.
— April 1999 in Littleton, Colo.: Columbine High School students Eric Harris and Dylan Klebold kill 12 classmates and a teacher and wound 26 others before killing themselves in the school's library.Obama said the Newtown horror obligated the nation to finally act to reduce gun violence. He wanted Congress to:
—Extend federal background checks to almost all gun sales.
—Pass a new, stronger ban on the sale on some semi-automatic rifles considered "assault weapons."
—Ban the sale of ammunition magazines with more than 10 rounds.
—Provide money to help more schools add police officers, psychologists, social workers or counselors.
—Help more people get mental health care.
Obama also made some changes by executive order, including:
—Steps to encourage states to submit more data to the federal background check system.
—Directing government agencies to study causes and prevention of gun violence. A law banning the use of federal money to "advocate or promote gun control" had squelched federal research.The major components of Obama's plan — background checks, the assault weapons ban, the limit on ammunition magazines — were quashed by the Senate.
Some members of his own Democratic Party, which controls the Senate, opposed the measures. Republicans were nearly united against them.
At a news conference with the president, Mark Barden, who lost his 7-year-old son, Daniel, in the Newtown shooting, said the families would return home "disappointed but not defeated."
Obama urged voters to pressure Congress on the issue. "This effort is not over," he said, and change will come "so long as the American people don't give up on it."Public support for tightening gun laws has dropped off as the Dec. 14 school shooting slips further into the past.
One month after the Newtown attack, 58 percent of Americans said they supported stricter gun laws, an AP-GfK poll found. This month, support was 49 percent.
Some specific gun proposals still have strong appeal, however, polling shows.They're hugely popular: More than 8 in 10 Americans support requiring background checks for buyers at gun shows, according to the AP-GfK poll in January. So closing the "gun-show loophole" looked like a potential place where gun control Democrats and gun rights Republicans might agree.
Two senators, Patrick Toomey, R-Pa., and Joe Manchin, D-W.Va, tried to bridge the divide with a compromise that would subject buyers at gun shows and on the Internet to the checks but exempt noncommercial transactions like sales between friends and family. On Wednesday their measure was supported by a majority of senators but fell short of the 60 votes needed to advance.
The existing system, created under the 1993 Brady law, requires licensed gun dealers to submit a buyer's name before completing the sale. Convicted criminals and people who have been declared by a judge to be "mentally defective" are among those barred from buying a gun. Private gun owners and sellers at gun shows don't have to run the federal checks.
It's unclear how many buyers avoid scrutiny this way. Gun control advocates often claim that about 40 percent of guns are sold without the checks. But that's based on a survey from 20 years ago, when the background check system was just starting, and it was considered a rough approximation.
A few states have their own, more comprehensive background check requirements.Another area of public agreement: Eight in 10 Americans want more done to prevent people who are mentally ill from buying a gun, according to Pew Research Center polling.
One way is to do a better job of getting treatment to people who need it. Obama wants to spend $235 million in federal money to identify and treat mental illness, especially in young people, and to study how to prevent shootings. The idea has appeal on both sides of the gun control debate, as well as among advocates for the mentally ill, although they stress that most people who need care aren't violent.
U.S. law bans gun sales to people who have been involuntarily committed or formally found to be dangerously mentally ill by a court or similar authority. But the federal background check system is weakened by paltry information from some states. Obama says he will do more to encourage states to share their mental health records.
A vote on a bipartisan proposal for improving mental health programs was set for Thursday in the Senate.One of the most-discussed gun control ideas — reviving the 1994 ban on sales of "assault weapons" — couldn't get anywhere in Congress.
A majority of Americans — 55 percent — surveyed for the AP-GfK poll said they favored a ban on military-style, rapid-fire guns; about a third opposed it. Feelings run strong on both sides. Backers of gun rights are especially active lobbyists, however. These guns are popular with recreational shooters and people who consider them a menacing choice for home defense.
Other knocks against the proposal: Defining an "assault weapon" has always been tricky, and there's scant evidence that the old ban worked.
Under the now-expired law, some semi-automatic rifles and pistols were banned by name, including the Uzi, the AK-47 and the Colt AR-15, which is similar to the military's standard issue M16. Others were banned because they had a combination of characteristics listed in the law.
Manufacturers simply skirted the ban by producing guns under new names or by making simple design changes.
The 1994 law wouldn't have covered the military-looking Bushmaster .223 rifles used in the Colorado theater and Connecticut school shootings, even if the ban were still in place. The old law did apply to another aspect of those shootings — high-capacity magazines.The Senate also rejected Obama's call to revive the expired ban on sales of magazines that hold more than 10 rounds of ammunition.
These devices feed bullets into the firing chamber automatically for rapid shooting. The larger the capacity, the more bullets a shooter can fire without pausing. In an attack, a killer who stops to reload might give victims a chance to flee or fight back.
Police said Adam Lanza came to Sandy Hook Elementary with several 30-round magazines and fired more than 150 shots. The shooter in the Colorado theater used a 100-round magazine, police said.
Half of Americans support a ban on the sale of high-capacity magazines, according to the AP-GfK poll. Nearly 4 in 10 oppose it.The National Rifle Association, the lead group lobbying against gun control, wants to keep the focus on protecting students at school.
A task force created by the gun rights group recommends that schools use specially trained armed guards or police officers to protect students, if the local community agrees. NRA Executive Vice President Wayne LaPierre says, "The only thing that stops a bad guy with a gun is a good guy with a gun."
The NRA task force also called for private funding and federal grants to help pay for increased security. And it says schools need to improve their ability to assess threats and handle warnings of violent or antisocial behavior by students.
Obama has proposed spending more federal money to help schools improve safety by adding specially trained police officers and counselors and improving safety planning.
The public is about evenly divided on the idea of requiring an armed guard in every school, according to a Washington Post/ABC News poll in March.Obama says he wants to address not only mass shootings but also "the epidemic of gun violence that plagues this country every single day."
The United States logs more than 30,000 gun deaths per year. The majority — about 6 out of 10 — are suicides. In calling for better gun control, Senate Majority Leader Harry Reid, D-Nev., has talked about the death of his father, who committed suicide with a gun in 1972.
Homicides account for more than 10,000 deaths each year, according to National Center for Health Statistics data through 2009. Roughly 2 out of 3 murders are committed with a gun, FBI statistics show.
Gun advocates note that federal statistics don't capture how many lives are saved when people use firearms to protect themselves — a number that researchers have found difficult to pin down.
The overall rate of violent crime, including crimes with firearms, has dropped sharply over the past two decades in the U.S.It's the Constitution's Second Amendment that guarantees a right to guns.
It says, "A well-regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed."
Exactly what that means has been debated for decades. In 2008 the Supreme Court ruled that it gives individuals a right to own firearms, even if they're not in a militia. But the justices also have signaled that some regulation of guns can be constitutional.
Which laws pass muster? The court has yet to say. Several states have gun restrictions that are tougher than U.S. law and are being challenged in court. Those cases should lead to more clarity.
Meanwhile, Connecticut, New York, Colorado and Maryland are tightening their gun laws in response to the recent mass shootings.
The January AP-GfK poll found 51 percent of Americans felt laws limiting gun ownership infringed on the public's right to bear arms; 41 percent said they do not.About a third of Americans say someone in their household owns a gun, according to an AP-GfK poll. But gun ownership is declining. Back in 1977, about half of households had a gun, the General Social Survey found.
Protection is the top reason people give, according to a Pew Research Center poll. About half of gun owners cited safety; about a third said hunting.
Every state but Illinois and the District of Columbia issue permits to allow people to carry concealed weapons under certain conditions. For example, the gun owner might need to pass a background check first. Some states require safety classes or a license that's hard to get. The Senate on Wednesday rejected a Republican plan to make states recognize the permits issued by other states.
Federal laws prevent the government from tracking how many guns are sold every year and who buys them, so there are no definitive statistics.
Roughly 310 million guns were owned or available for sale in the United States in 2009, according to a study by the nonpartisan Congressional Research Service. That's about 114 million handguns, 110 million rifles and 86 million shotguns.