Columbine, Virginia Tech, Sandy Hook, and now, a community college in Roseburg, Oregon. One after another, mass shootings have horrified the nation, stoking debate about the availability of legal guns and anguish over the inability of society to keep weapons out of the hands of seething killers.

But such rampage killings are not the typical face of gun violence in America. Each day, some 30 people are victims of gun homicides, slain by rival gang members or drug dealers, trigger-happy robbers, drunken men after bar fights, frenzied family members or abusive partners. An additional 60 people a day kill themselves with guns.

In Chicago alone in September — the city’s deadliest month in recent years — there were 57 homicides, most by gunfire, and 351 were shot and wounded. In total, counting suicides, 33,636 people in the United States were killed by firearms in 2013, according to the latest federal data.

"Mass shootings focus the public’s attention," said Garen J. Wintemute, director of the Violence Prevention Research Program at the University of California, Davis, School of Medicine. "But we lose on the order of 90 people a day to firearms. We need to keep our eyes focused on the larger picture."

Yet there is bitter disagreement over how to respond in this gun-saturated country, and especially in the current political campaign, over whether expanded, tougher background checks would make a difference.

Complicating any solutions is a stark reality about the origins of many of the guns used in crimes. Most of the estimated 300 million guns in civilian hands in the United States — kept by at least a third of U.S. households, according to a 2014 survey by Pew Research Center — were bought legally. But few criminals obtained their firearms that way, turning instead to an underground market.

In the largest federal survey of prison inmates on the subject, done in 2004, only about 10 percent of convicts who had carried guns said they bought them from licensed dealers. Most said they bought them from, or traded with, relatives, friends or street acquaintances such as fences, drug dealers and gang members.

Gun theft is a major source of such weapons. Evidence suggests that at least 250,000 guns are stolen in home and store burglaries each year. Some criminologists say the number may be significantly higher.

Once weapons start circulating in this underworld, they tend to change hands frequently, said Philip J. Cook, a professor of public policy at Duke University.

Two years ago, Cook questioned 100 prisoners in the Cook County, Ill., jail about how they obtained weapons. Some of the Chicago-area inmates said they had purchased from traffickers from another state, or sent fellow gang members to Indiana or other states where gun laws are looser. Many of those questioned stressed the primacy of family, friends and fellow gang members as sources of guns. They said they were reluctant to deal with strangers, fearing a police sting or the purchase of a "dirty" gun that could link them to a crime.

In some gangs, the inmates said, gun sharing is common; 15 youths in a neighborhood might have access to four guns, as needed. Guns may also be given as gifts to friends or comrades getting out of prison.

Conservative opponents argue that controls on legal firearm sales cannot directly keep firearms away from criminals.

Yet applying background checks to private gun sales as well as commercial ones, with stronger criteria for denying purchases, remains a top goal of many gun control advocates and scholars who study firearms violence. They point to major gaps in the current system of checks as well as evidence that extending checks to private transactions can slow the flow of weapons into the underground market.

The issue has flared up in the presidential campaign. President Barack Obama and some Democrats — Hillary Rodham Clinton among them — have called for universal background checks as an important step. Many of the Republican candidates join pro-gun groups in arguing that such rules will hinder only law-abiding citizens.

A primary concern of those calling for expanded checks is the absence in most states of any vetting procedure when a gun is purchased from a private party — a friend, a seller advertising online, a small-scale seller at a gun show. By federal law, background checks are required only for purchases from licensed dealers; people with felony records or certain official records of mental illness are barred from buying.

A significant minority of guns are acquired legally but without background checks, which many authorities call a worrisome loophole. In a national survey of more than 2,000 gun owners, conducted this year by the Harvard School of Public Health and not yet published, 40 percent of owners said they had acquired their most recent firearm without a background check. While in some cases these guns were inherited or given by relatives, most of them were purchased, said Deborah Azrael, one of the study’s leaders.

Eighteen states and the District of Columbia have established their own checking system and also applied it to private handgun transactions.

Daniel Webster, an expert on gun violence at the Johns Hopkins University Center for Gun Policy and Research, cites two recent examples as evidence that expanded background checks can affect gun markets and violence.

In 2007, Missouri ended a decades-old system of background checks and licensing for handgun purchases, including private sales. According to research by Webster and colleagues, the change quickly led to an increase in gun diversions to criminals and to a 25 percent increase in firearm homicides over the three years that followed, while homicides committed by other means did not rise.

In contrast, Connecticut in 1995 extended background checks to private sales and established a handgun permit system. Over 10 years, the rate of gun murders fell by 40 percent.

"There is a connection between regulating the formal market and the number of guns that enter the underground market," Webster said. A large share of violent criminals, he added, are "dirt-poor," and to them, price matters.

Still, general controls on gun sales may do little, by themselves, to block a determined mass killer.

In the shooting last week in Oregon, in which Christopher Harper-Mercer killed nine people, all 14 of the guns available to him — either used in the attack or left at home — were bought legally by him or by a relative from a licensed dealer.

"This is the sort of killing you’re least likely to prevent with gun control laws, partly because the killers are so motivated," said Gary Kleck, a criminologist at Florida State University.!~neIP~!

But a new initiative in California is directed specifically at such people. In a law that will take effect in January, family members or police will be able to ask a judge for a temporary gun violence restraining order if they see someone in an ominous emotional spiral, threatening violence and perhaps collecting weapons.

Disputes over civil liberties appear likely. But with legal procedures modeled on restraining orders for domestic violence, the law says, officials could obtain a search warrant and seize the person’s guns for a brief period, pending evaluation.

The idea builds on "threat assessment" efforts by some schools and police departments, which focus on people seen as threatening. Support for the idea grew after a deadly rampage in Isla Vista, California, last year.

Before the rampage, the family of the gunman, Elliot O. Rodger, had feared that he was becoming dangerous and had even notified law enforcement. Officers visited him but saw no evidence of mental illness that would warrant taking any action. They did not check his gun purchase records or search his home.

"If they had searched, they’d have found not only three guns, but 40 loaded magazines," said Wintemute of the University of California, Davis.

"That would have just screamed, ‘Trouble coming!’" he said.