A man pulled out a gun on a crowded train multiple times before he shot and killed a passenger; no one noticed the gunman until the shots were fired.

Officials told the San Francisco Chronicle Nikhom Thephakaysone, 30, allegedly killed 20-year-old Justin Valdez in a "random encounter." 

It is common for passengers to be on their smartphones and tablets during their commute.  However, authorities believe Thephakaysone was "hunting" for a stranger to kill, but people were oblivious to the danger in front of them because of the devices.

"These weren't concealed movements - the gun is very clear," District Attorney George Gascón told the Chronicle. "These people are in very close proximity with him, and nobody sees this. They're just so engrossed, texting and reading and whatnot."

Thephakaysone has pleaded not guilty to the charges against him, including murder and illegal possession an assault weapon.  San Francisco Police Chief Greg Suhr told the Chronicle people have become so absorbed in their technology that they become unaware of what's going on around them.

"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," Suhr said. "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.' "

However, authorities acknowledged smartphone cameras have become tools to capture incidents, later becoming evidence in their investigations to 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."