The beam from the intruder's flashlight pierced the blackness of the bedroom at 4:45 a.m., sweeping across the down comforter and into Eric Martin's eyes. Outside, the streets of his Utah subdivision lay still and silent.
But as Martin rolled to the floor, reached into the nightstand drawer and drew out his 9 mm pistol, the 46-year-old executive's mind raced with calculation: Would this man harm Martin's fiancee or her son? Was an accomplice outside waiting? What if he pulled the trigger and hit the sleeping 8-year-old across the hall?
In the weeks since the Connecticut school massacre, some of the most intense debate has swirled around how to keep guns from criminals without infringing on the ability of lawful gun owners, like Martin, to protect themselves and their families.
Indeed, protection is now the top reason gun owners cite for having a firearm, a new survey shows, a figure that has nearly doubled since 1999.
But even after years of study, there is little clarity on how, exactly, Americans use guns to protect themselves in moments of jeopardy - or how often. Researchers known for sharp disagreement on the self-defense riddle say the answers may be shifting dramatically because of a steep drop in crime, an increase in guns and state laws giving owners more leeway to wield them.
Determining the absolute value of guns for self-defense is clouded by that complex dynamic of policy, judgment and circumstance. Still, both advocates of gun rights and of gun control understand the issue's importance in shaping the debate.
"When there's a threat outside your door, the police aren't going to be there ... the guys trained to save lives aren't going to be there," said Dom Raso, a commentator for the National Rifle Association's online news channel, in a video posted recently by the gun rights group.
And even while calling for new gun laws, President Barack Obama, too, acknowledged the legitimacy of self-defense in an April 8 speech in Hartford, Conn., when he recounted a conversation with his wife, Michelle, after campaigning in rural Iowa.
"Sometimes it would be miles between farms, let alone towns," Obama said. "And she said, `You know, coming back, I can understand why somebody would want a gun for protection. If somebody drove up into the driveway and, Barack, you weren't home, the sheriff lived miles away, I might want that security.'"
With Americans split over whether guns more often save lives or jeopardize them, researchers have long parsed surveys of crime victims done in the 1990s, arguing over what the numbers mean.
But since then, crime has plummeted in the U.S. The rate of violent crimes including murder and assault fell by nearly half from 1992 to 2011, while the rate of reported property crime dropped 41 percent, data compiled by the Federal Bureau of Investigation show.
That drop has researchers considering the possibility, even the likelihood, that many fewer Americans are drawing firearms to protect themselves.
"I'm pretty confident that whatever the number is, it did go down ... because overall crime went down," said Gary Kleck, a Florida State University criminologist whose 1990s research, widely cited by gun rights activists, concluded that Americans drew their firearms in self-defense up to 2.5 million times a year. That translates to about 3 percent of all gun owners during the course of a single year.
But the drop in crime means there are far fewer occasions now for Americans to use guns for self-protection, Kleck said, making it likely that the number of annual self-defense usages of guns "should be about half as big now as they were back then, 20 years ago."
Even if such a drop were documented, it would still leave a scenario of relatively widespread use of guns for self-defense suggested by Kleck far at odds with research done by his critics.
The most outspoken has long been David Hemenway, director of the Harvard University Injury Control Research Center. He contends Kleck's survey vastly overinflates the number of times people use guns to defend themselves - for example, by estimating thousands during the course of break-ins, though many of those homeowners either didn't own guns or remained asleep during the crime. Kleck, in turn, says Hemenway and others depend on surveys that significantly undercount self-defense gun use.
Hemenway, also relying on 1990s surveys, concluded Americans were then wielding guns for self-defense about 200,000 times annually.
Others researchers, analyzing the federal government's National Crime Victimization Survey, say the number of times guns were drawn for self-defense was even lower, about 80,000 times a year.
But Hemenway, too, is rethinking his estimate. If declining crime was the only change, he said, it would be reasonable to expect a parallel decrease in the number of times Americans use guns to defend themselves.
"You certainly have less opportunities to use a gun in self-defense appropriately," he said. "The problem is, over long periods of time, so many other things may be changing."
Since the 1990s, 18 states have passed stand-your-ground laws. At the same time, many more states eased the ability of gun owners to legally carry concealed weapons. The number of guns Americans own has also jumped to about 300 million, although researchers say the percentage of households with guns has declined.
Today, more gun owners than ever - 48 percent according to a March poll by the Pew Research Center - cite self-protection as their primary reason for having a firearm. That has nearly doubled since 1999, and now far surpasses the declining number of gun owners who say they own a firearm primarily for hunting.
The figure confirms personal security as a major concern for most Americans, reflected in attitudes about guns, said Michael Dimock, director of Pew's political polling unit.
"On both sides of this, the safety issue is front and center," he said. "For most people, this is not a casual choice. There's a sense of safety that gun owners associate with having that gun and there's a clear sense of risk that non-gun owners associate with guns."
Demands for gun control have led many gun owners to point to the value firearms play in allowing Americans to protect themselves, a position Martin, the Utah homeowner, agrees with based on firsthand experience.
When a robber broke into his home in St. George before dawn in late March, he and fiancee Rachel Cieslewicz were in bed. Her son, Canyon, was asleep in a room across from their doorway.
Martin says he trained to use a gun when he was in his early 20s and has long kept one for protection. Before the break-in, he had thought carefully many times about how, precisely, he would react in such a situation.
But in the dark, gun in hand, judgment and action were instantaneous. Even with the intruder directly in front of him, Martin says he realized he could not fire because of the chance the bullet might pierce the wall and hit the sleeping 8-year-old.
"I knew I needed to protect Rachel. I knew I needed to protect Canyon," he says.
When Martin pulled back on the gun's slide to load a bullet into the chamber, the man in the doorway bolted and Martin gave chase. Seconds later they were outside, but as the robber tried to escape, he tripped and fell. Martin, taking position behind a wall, trained his gun on him and ordered him to stay down, threatening to shoot when the man moved. That's where they were when police cruisers arrived.
In the days since, scores of people, including police, have commended Martin for his cool-headed reaction. Martin and Cieslewicz say they have no doubt about the gun's self-defense value. But he acknowledges the complexity of the calculus.
"What would have happened if the guy hadn't fallen, tripped over the stuff he was stealing, and I hadn't gotten him pinned down?" he said. "Or if he'd run down the street 50 feet in front of me," and Martin had opened fire?
"Is that self-defense or is that me just trying to let off a little bit of steam at that point? That changes the whole dynamic of everything."
Multiplying that uncertainty by the many confrontations involving a gun where the roles of the players are less clear helps explain researchers' disagreement about the use of gun play in self-defense.
Kleck, of Florida State, said that when people are surveyed correctly, the vast majority only disclose clear-cut incidents where they were in the right and guns were used correctly to protect their own and their families' lives.
But Hemenway, the Harvard researcher, says many of the incidents people characterize as self-defense are dubious.
"We expected pretty brave and wonderful things," he says, about a 1990s survey of gun owners. "But most of the things that were presented (as self-defense) were little more than escalating arguments. It wasn't like this is a good guy and this is a bad guy. It's two people who got into an argument and somebody drew a gun."
One much-debated self-defense claim is in the February 2012 death of Trayvon Martin, an unarmed 17-year-old killed in a Florida subdivision by a neighborhood watch captain.
But many other incidents, highlighting the role of a gun and quick thinking in protecting lives, don't get the attention they deserve, said Daniel Terrill, an editor of Guns.com. The Winnetka, Ill.-based website regularly publishes accounts of self-defense.
Then there are reports of gun usage in which people seem either to have used questionable judgment or sought out a dispute - cases that elicit fierce discussion among gun owners on the site.
"It's a debate piece for them and ... they will go over that ad nauseum, saying it was justified or it wasn't justified. So you find a lot of these stories, actually, are kind of morally gray," Terrill said. The uncertainty reflects both the complexity of decision-making involved in gun use, as well the many unknown circumstances that led to a confrontation in the first place, he said.
The increased focus by gun owners on self-defense while the threat of crime decreases reflects a long-standing disconnect in public perceptions of violent crime, said Mark Warr, a University of Texas criminologist.
"Americans don't know that the crime rate has been going down," said Warr, noting that public perception is shaped by television crime dramas and news reports focusing on the most violent offenses. "What happens is that people watch this dangerous image of the world and they buy into the idea that the world is a really, really dangerous place."
Public fears spiked in the 1960s in response to a substantial increase in crime, reflected in increased purchases of guns, homes in gated subdivisions and security systems, he said, and concern about crime has never eased to pre-1960s levels, even though crime has steadily declined.
But trying to figure out how those safety concerns, attitudes regarding gun ownership, changes in law and other factors are affecting the use of guns for self-defense remains difficult.
David McDowall, a professor in the school of criminal justice at the University at Albany, State University of New York, said that given all the changes in law and gun ownership, it is quite possible that a greater proportion of people now draw firearms in self-defense.
"I think that's really the interesting question," he said.
And gun owners point out that the decision to draw a weapon in self-defense when confronted by an intruder makes the abstracts studied by researchers and policymakers all too real. To Cieslewicz, it comes down to recalling her fears for her son's life as a stranger loomed in her bedroom doorway.
"It was," she said, "an absolute moment of terror."