SAN FRANCISCO, Calif. -- A man standing on a crowded Muni train pulls out a .45-caliber pistol.
He raises the gun, pointing it across the aisle, before tucking it back against his side. He draws it out several more times, once using the hand holding the gun to wipe his nose. Dozens of passengers stand and sit just feet away -- but none reacts.
Their eyes, focused on smartphones and tablets, don't lift until the gunman fires a bullet into the back of a San Francisco State student getting off the train.
Investigators say this scene was captured by a Muni camera on Sept. 23, the night Nikhom Thephakaysone, 30, allegedly killed 20-year-old Justin Valdez in an apparently random encounter.
For police and prosecutors, the details of the case were troubling -- they believe the suspect had been out "hunting" for a stranger to kill -- but so too was the train passengers' collective inattention to imminent danger.
"These weren't concealed movements -- the gun is very clear," said District Attorney George Gascon. "These people are in very close proximity with him, and nobody sees this. They're just so engrossed, texting and reading and whatnot. They're completely oblivious of their surroundings."
Gascon said what happened on the light-rail car speaks to a larger dilemma of the digital age. As glowing screens dominate the public sphere, people seem more and more inclined to become engrossed, whether they are in a car or a train or are strolling through an intersection.
"When you used to go into a public place, you assumed everyone was in that place with you," said Jack Nasar, an Ohio State University professor in city and regional planning who specializes in environmental psychology. "What happens to public places when everybody is talking on a cell phone? Everyone is somewhere else.
"Someone can take a gun, hold it up, and nobody will notice it."
Nasar has been studying the dangers of cell phone distractions for nearly a decade. For a 2008 study, he and his research team planted objects such as a sign reading "UNSAFE!" and fake vomit on a stretch of sidewalk, and instructed 60 people to walk the path.
Those talking on cell phones were far more likely to miss the cues, he said, results that mimicked the findings of studies of distracted drivers.
The implications are nothing new for Bay Area police officials, who say smartphone thefts have become an epidemic not only because the devices are valuable, but because the victims are preoccupied.
"Oftentimes when you interview people who get their phones stolen, when you ask them to describe where the person came from, what he was wearing, they have no idea," said San Francisco Police Chief Greg Suhr. "It's not uncommon to read in a police report that a person 'came out of nowhere' or 'I didn't see where he came from.' "
Suhr said 2 out of 3 robberies in the city involve smartphones.
Phones can help, hinder
Both Suhr and Gascon want to strike a balance in the use of technology. They acknowledge that the ubiquity of smartphones -- the videos and photos they capture, and the quick communication they allow -- often helps catch criminals.
"I'm not going to say we don't appreciate the cell phone videos that we have gotten on so many occasions that have helped us solve crimes," Suhr said. "But it makes people so incredibly vulnerable to crime. And the inattention, which creates this tremendous vulnerability to people, is just something that's so easily corrected."
The advice from police is simple -- pull out cell phones less, pay attention more. It's a mantra San Francisco State police officers have been spreading around campus since the Sept. 23 shooting, said student Jordan Sanchez, 17.
Sanchez spoke as he waited for a Muni train, after pausing to look up from his phone. Like many passengers around him, he said he didn't think the way he used the device in public put him in danger.
"When you're bored and there's nothing to do, you go on your phone," he said.