SAN FRANCISCO (AP) -- The young man, just 20, stepped off the train. Another man followed him, pulling out a gun. There was a single shot, startling the preoccupied commuters. Justin Valdez fell, gasping and screaming for help as he died, a bullet to his head.
That's according to authorities who have reviewed footage from a surveillance camera on the San Francisco Municipal Railway car on Sept. 23.
And what those videos also show, authorities say, is that the suspect Nikhom Thephakaysone, 30, had already pulled out his gun three of four times while aboard the crowded car, but the dozen passengers aboard were so hypnotized by their phones and tablets they failed to notice; not when he pointed the .45-caliber pistol across the aisle, nor even when he wiped his nose with the hand holding the gun.
"There are people facing in his direction, you can see them on smartphones and their tablets, highly distracted," District Attorney George Gascon told The Associated Press on Wednesday. "And you can see the suspect pulling the gun with his right hand, bringing it up to his face at least three or four times, parallel to his face and no one sees this going on."
District Attorney's office spokesman Alex Bastian said some passengers have told investigators they didn't know the suspect had a gun until they heard a gunshot. And they said after the shooting, the train's doors closed and it started to move; passengers alerted the driver and it slowly came to a stop, he said.
Bastian said that some passengers told investigators that they were looking down while on the train, but they did not say specifically whether they were looking at their mobile devices.
But Bastian reiterated that on the surveillance video it appears that many passengers were looking at their phones or tablets.
Gascon said that while he's not suggesting if people had been more aware, the shooting could have been prevented, the incident highlights the dangers of increasingly living in a virtual world, with less human interaction.
"We know from all of these cellphone robberies, many times the victims will tell you that they never saw the assailant because they're too busy texting and talking," Gascon said. "We have this whole behavior taking over people when they are using these devices because they are not paying attention to their surroundings."
Today's devices are far more absorbing than newspapers and boom boxes of the past, says Los Angeles-based behavioral health expert Rob Weiss, who studies the effect of technology on society,
"It's not like reading a book or reading a magazine or even watching television," he said. "Today's devices require a constant stream of interaction, and that requires much more brainpower than passive watching or listening."
Experts and bloggers weighed in Wednesday, as news that the trainload of commuters hadn't noticed the gun spread.
American University linguistics professor Naomi Baron says that part of being a civilized human being is relating to those around you.
"If you're sitting in a public space such as a bus and something happens, you have a social obligation to be part of a community," she said. "Adults are now so entrenched in using their devices, there's something wrong with our values."
New York psychotherapist Jonathan Alpert, who is increasingly seeing issues related to digital devices, is even more blunt: "Texting and reliance on devices has bred a culture of social retardation."
Authorities released surveillance footage prior to the shooting in which Thephakaysone rocks a bit with the rattling train, his back leaning against doors. He pulls a phone from his loose, white jacket pocket and checks if for a few seconds, adjusts the brim of his dark baseball cap, and at one point appears to laugh randomly to himself.
Authorities have not released the rest of the footage they say shows him pulling out the gun, citing an ongoing investigation. But several officials described what they saw on the surveillance video.
In the video, Thephakaysone is seen stepping out of a Muni train and extending his arm, followed by the sound of one gunshot, said Alex Bastian, a spokesman for the DA's office.
"And then you hear the victim gasping and screaming for help," Bastian said.
During a vigil days after the murder, Valdez's sister Jessica Labidi said witnesses told her that when he was shot he screamed for help.
"Everyone just came running by his side, they were holding his hand," she said.
Thephakaysone fled after the shooting, police said, but his gun was found at the scene and he was arrested a day later at his home, with $20,000 cash, two assault weapons and combat knives.
Both men were students at San Francisco State University, spokeswoman Emily Griffin said.
Valdez was a full time sophomore, a promising student who was mostly raised by his grandmother and uncle. He swam on his high school swim team in Garden Grove, Calif., and worked in San Francisco as a lifeguard.
Thephakaysone had entered San Francisco State University in 2008 as a transfer student, but was only there full-time for that year, studying to be a dietitian. He was taking just one course through the school's Open University program that allows non-enrolled community members to take classes.
Prosecutors said that before he killed Valdez, Thephakaysone stuck a handgun in another man's back but did not pull the trigger.
Authorities say there's no indication the two knew each other, and said they're trying to find out why he might have gathered a cache of weapons and gone hunting for a victim. Thephakaysone had no prior run-ins with authorities, police said, and no apparent criminal or civil court records.
Thephakaysone has pleaded not guilty to charges including murder and assault with a semi-automatic handgun. He is being held without bail and is due back in court on Nov. 20.