The San Francisco Police Department said security footage from the train car showed Nikhom Thephakayson pulling out his .45-caliber pistol, raising it and pointing it across the aisle multiple times.
At one point he even used the pistol to scratch his nose, The Huffington Post reported Wednesday.
"These people are in very close proximity with him, and nobody sees this," District Attorney George Gascon told the San Francisco Chronicle. "They're just so engrossed, texting and reading and whatnot. They're completely oblivious of their surroundings."
No one noticed the man until he pulled the gun out again and fired a bullet into the back of another passenger, San Francisco State University sophomore Justin Valdez, 20, killing him, The Huffington Post reported. Police said they believe the shooting was a random, unprovoked incident.