Security footage allegedly shows Nikhom Thephakaysone, 30, waving a gun and pointing it across the aisle several times on the San Francisco-based train on Monday, each time failing to the draw the attention of any passengers.
Thephakaysone allegedly then opened fire on a random passenger, killing 20-year-old college student Justin Valdez.
"These weren't concealed movements — the gun is very clear," District Attorney George Gascon told the San Francisco Chronicle. "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."