Joe Zamudio was out buying cigarettes Jan. 8 when he heard what sounded like fireworks but quickly realized were gunshots. He reached into his coat pocket for the 9-millimeter pistol he carried, clicking the safety off.
He heard yelling around him: "Shooter, shooter, get down!"
Zamudio saw a young man squirming on the ground and an older man standing above him, waving a gun.
Zamudio, 24, had his finger on the trigger and seconds to decide.
He lifted his finger from the trigger and ran toward the struggling men.
As he grabbed the older man's wrist to wrestle the gun away, bystanders yelled that he had the wrong man - it was the man on the ground who they said had attacked them and U.S. Rep. Gabrielle Giffords, D-Ariz. The gun the older man was holding had been wrestled away from the shooter. Police later identified 22-year-old Jared Lee Loughner as the suspect.
"I could have very easily done the wrong thing and hurt a lot more people," said Zamudio, who helped subdue the suspect until authorities arrived.
The fact that Zamudio was carrying a gun, and his split-second decision to keep it in his pocket, has come to encapsulate the complexity of the national gun debate.
Gun-rights advocates say his quick action showed that a well-armed - and well-trained - person could protect himself and the public.
But gun-control advocates see Zamudio's story as an example of how Arizona's gun-friendly culture and lax gun laws have not only failed to make the streets safer, but also have potentially endangered lives.
"They always say, 'What if someone with a concealed weapon was there and could stop this,' " said Kristen Rand, legislative director for the Washington-based Violence Policy Center. "Well there was, and he almost shot the wrong person."
As for Zamudio, he said he was glad he had his gun that day and knows he did the right thing, even if he was not able to stop the shooting.
"I wish I had stopped him sooner," Zamudio said. "We're all responsible to help."
At Black Weapons Armory, a family-owned gun shop in Tucson, former Marine Phil Davis and others said they are glad they live in a state where it is legal to carry a gun.
"Arizona has very common-sense gun laws that allow me to protect my family," said Davis, 36, a technology worker, as his two sons fiddled with a rifle scope nearby.
Jeff Prather, a former Green Beret who runs a firearms school at the shop, marveled at Zamudio's composure amid so much confusion. He said most casual gun owners would freeze or focus so tightly on the threat that they would barely notice anything else.
"You think that people could perform like that if they didn't have gun familiarity?" Prather, 53, asked. "That's freedom, and Arizona is leading the way."
Zamudio is definitely ex-military, he figured. Those types of instincts don't come easy.
In fact, Zamudio had no formal firearms training. His father, a prison guard and Vietnam veteran, taught him to shoot as a boy in the desert outside town. When his father died five years ago, he left Zamudio an antique revolver and one rule to live by as a gun owner: "Pray you never have to use it, but be prepared to use it if you have to."
Zamudio said he has been carrying a gun since he was 19, and prefers to keep it concealed so as not to make people uncomfortable. He got a deal last summer on the Ruger P95 he had on him during the shootings.
Looking back, Zamudio sees his years of target practice as preparation for that Saturday morning. With his Ruger in hand, he instinctively ran toward the shooter instead of away.
As he closed in on the two struggling men, he heard the older man with the gun shouting: "I'll kill you!" Still, he resisted the temptation to fire.
It was then that he recognized the Glock 19 in the older man's hand. Zamudio had used that type of weapon before and noticed a key detail: The slide on top of the gun was locked open, indicating it was in the process of being reloaded and wasn't quite ready to use. "I knew he couldn't shoot me," Zamudio said.
Knowing that, he charged forward without shooting. "I decided that I wasn't the one to kill him," he said.
He slammed into the older man and held him against a wall until bystanders told him he had the wrong guy.
Then, he and the older man, whose name Zamudio never learned, turned their attention to the younger man, holding him down with the help of two others.
As the suspected shooter squirmed, Zamudio again thought about pulling out his gun. "But with all those people around, I didn't want more hysteria," he said.
So he kept the gun in his pocket.
Zamudio wonders if he could have done anything differently. His one regret is that he did not reach the shooter and stop him sooner.
After a change in Arizona law last year, no permit is required to carry a concealed gun nearly all of the time. Whether such gun laws have made the state safer is a matter of debate.
Both sides have trotted out reams of statistics to prove their point with little effect in resolving the debate.
John Lott, a Fox News commentator who wrote "More Guns, Less Crime," said some studies have shown that states with the greatest increase in gun ownership also saw the greatest drop in crime.
Opponents note that Arizona ranked ninth in the nation for gun deaths, according to a 2007 study by the federal Centers for Disease Control and Prevention. Rand, the gun-control advocate, said more guns in the hands of the populace drive up gun crime rates when domestic disputes, traffic stops and workplace arguments escalate to shootings.
"People that live in states like Arizona that think guns are the answer to their problems will continue to go down that road and ultimately pay the price," she said.
National research, including studies by the National Academies of Science, has not come to a clear conclusion about whether allowing more concealed weapons makes communities safer. Experts say it is nearly impossible to tease out how one factor affects something as complex as crime.
"It's easy to say we have an easy fix here: All we need is more law-abiding citizens carrying guns," said John J. Donohue III, a law professor at Stanford University. "But in the real world, it's not that clear-cut."
Zamudio admits being conflicted about some of Arizona's more extreme gun legislation, including a proposed law allowing college faculty and students to carry concealed guns on campus. State legislators plan to take up the issue this month.
He imagined students might get angry during a debate about some controversial topic like religion and draw their guns.
But at the same time, he worried about a replay of the shooting rampage at Virginia Tech in 2007 when a student killed 32 classmates and faculty before committing suicide.
"There can be the Virginia Tech guy pushing the door open and maybe you're sitting in the audience ready for him," Zamudio said. "It's all about who's holding them. The person holding the gun is the factor. It's not the gun."