Published: Oct 09, 2013 4:53 PM EDT

SAN FRANCISCO (AP) - Passengers on a crowded San Francisco commuter train were so absorbed in their phones and tablets they didn't notice the gunman until he randomly shot and killed a university student who had just exited the train, authorities said.
The man drew the gun several times on the train, with surveillance video showing him pointing it across the aisle without anyone noticing and then putting it back against his side, authorities said.
Before that moment, footage showed the man pull out the .45-caliber pistol and once wiped his nose with the hand holding the weapon, District Attorney George Gascon and Police Chief Greg Suhr said.
"On the surveillance video we have, everyone is looking at their cellphones and personal devices and nobody notices. I find that to be very concerning," said Gascon recently when announcing murder charges against Nikhom Thephakaysone in the Sept. 23 fatal shooting of Justin Valdez.
"It doesn't appear that anybody saw the gun when he actually took it out," Suhr told reporters when he announced that Thephakaysone was arrested the next day.
In the surveillance video, Thephakaysone is seen exiting halfway out of Muni train and then extending his arm out as if he had pulled out a gun, 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. A pistol was recovered at the scene.
Authorities declined a request by The Associated Press for the surveillance footage, citing an ongoing investigation.
Thephakaysone, 30, has pleaded not guilty to charges including murder in the attack on Valdez, 20, a student at San Francisco State University who was shot in the back of the head as he left the train on Sept. 23.
"This was a cold and calculating incident. This individual went out hunting that night and unfortunately, Mr. Valdez, was in the wrong place at the wrong time," Gascon said. "This was a wanton killing. There was absolutely no reason for this other than pure violence."
Thephakaysone also has been charged with assault with a semi-automatic handgun. Prosecutors said he stuck a handgun in another man's back earlier on the same night he shot Valdez as he looked for a victim.
He did not pull the trigger and went on the hunt again for a victim for more than an hour, authorities said.
"It almost seems like in the first instance, he didn't quite get the courage to pull the trigger," Gascon said. "And then in the second instance, he obviously did."
Thephakaysone was also charged with illegally possessing an assault weapon after police said they seized two such rifles, combat knives and razors at his home.
Police said he was also carrying $20,000 in cash when he was arrested.
"We have no idea why he did what he did. Right now, we have an unprovoked murder and there's no explaining," Suhr said. "You'd be hard pressed to understand why somebody would possess this sort of firepower and then be capable of carrying out a homicidal act."
Thephakaysone is being held without bail and is due back in court on Nov. 20.

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