More broadly, attempts by armed civilians to stop shooting rampages are rare—and successful ones even rarer. There were two school shootings in the late 1990s, in Mississippi and Pennsylvania, in which bystanders with guns ultimately subdued the teen perpetrators, but in both cases it was after the shooting had subsided. Other cases led to tragic results. In 2005, as a rampage unfolded inside a shopping mall in Tacoma, Washington, a civilian named Brendan McKown confronted the assailant with a licensed handgun he was carrying. The assailant pumped several bullets into McKown and wounded six people before eventually surrendering to police after a hostage standoff. (A comatose McKown eventually recovered after weeks in the hospital.) In Tyler, Texas, that same year, a civilian named Mark Wilson fired his licensed handgun at a man on a rampage at the county courthouse. Wilson—who was a firearms instructor—was shot dead by the body-armored assailant, who wielded an AK-47. (None of these cases were included in our mass shootings data set because fewer than four victims died in each.)

Appeals to heroism on this subject abound. So does misleading information. Gun rights die-hards frequently credit the end of a rampage in 2002 at the Appalachian School of Law in Virginia to armed "students" who intervened—while failing to disclose that those students were also current and former law enforcement officers, and that the killer, according to police investigators, was out of bullets by the time they got to him. It's one of several cases commonly cited as examples of ordinary folks with guns stopping massacres that do not stand up to scrutiny.

How do law enforcement authorities view armed civilians getting involved? One week after the slaughter at the Dark Knight screening in July, the city of Houston—hardly a hotbed of gun control—released a new Department of Homeland Security-funded video instructing the public on how to react to such events. The six-minute production foremost advises running away or otherwise hiding, and suggests fighting back only as a last resort. It makes no mention of civilians using firearms.www.readyhoustontx.govScreen shot: City of Houston video on mass shooters.

Law enforcement officials are the first to say that civilians should not be allowed to obtain particularly lethal weaponry, such as the AR-15 assault rifle and ultra-high-capacity, drum-style magazine used by Holmes to mow down Batman fans. The expiration of the Federal Assault Weapons Ban under President George W. Bush in 2004 has not helped that cause: Seven killers since then have wielded assault weapons in mass shootings.

But while access to weapons is a crucial consideration for stemming the violence, stricter gun laws are no silver bullet. Another key factor is mental illness. A major New York Times investigation in 2000 examined 100 shooting rampages and found that at least half of the killers showed signs of serious mental health problems. Our own data reveals that the majority of mass shootings are murder-suicides: In the 62 cases we analyzed, 36 of the shooters killed themselves. Others may have committed "suicide by cop"—seven died in police shootouts. Still others simply waited, as Holmes did in the movie theater parking lot, to be apprehended by authorities. Brownells.comDrum-style magazine for assault rifles

Mental illness among the killers is no surprise, ranging from paranoid schizophrenia to suicidal depression. But while some states have improved their sharing of mental health records with federal authorities, millions of records reportedly are still missing from the FBI's database for criminal background checks.

Hargarten of the Medical College of Wisconsin argues that mass shootings need to be scrutinized as a public health emergency so that policy makers can better focus on controlling the epidemic of violence. It would be no different than if there were an outbreak of Ebola virus, he says—we'd be assembling the nation's foremost experts to stop it.

But real progress will require transcending hardened politics. For decades gun rights promoters have framed measures aimed at public safety—background checks, waiting periods for purchases, tracking of firearms—as dire attacks on constitutional freedom. They've wielded the gun issue so successfully as a political weapon that Democrats hardly dare to touch it, while Republicans have gone to new extremes in their party platform to enshrine gun rights. Political leaders have failed to advance the discussion "in a credible, thoughtful, evidence-driven way," says Hargarten.

In the meantime, the gun violence in malls and schools and religious venues continues apace. As a superintendent told his community in suburban Cleveland this February, after a shooter at Chardon High School snuffed out the lives of three students and injured three others, "We're not just any old place, Chardon. This is every place. As you've seen in the past, this can happen anywhere."

Additional research contributed by Deanna Pan and Gavin Aronsen.