The day after the killing, 30-year-old Nikhom Thephakaysone was arrested at his home on murder charges, police said Wednesday.
Valdez was shot from behind after getting off an eastbound M-Ocean View Muni train at the corner of Randolph and Bright streets about 9:50 p.m. Monday, Sgt. Danielle Newman said. The attack might have been random, as police believe Valdez and Thephakaysone did not know each other.
Thephakaysone, according to police, is the person who appeared in Muni surveillance footage on the same light-rail vehicle that Valdez was riding in before he was shot. (Video can be seen here).
Newman said more information on the investigation will be revealed soon.
Loved ones of Valdez, who graduated two years ago from Garden Grove High School in Southern California, are stunned by the killing, saying he was the exact opposite of the kind of person who would feud with anyone.
"He was one of the most persistently positive kids I ever knew," said Steve Osborne, the principal of Valdez's high school. "Everybody is just devastated."
In their very last conversation, Osborne said he told Valdez he wanted his sons to grow up to be just like him.
Valdez was an accomplished swimmer and water polo player and was president of his high school's environmental club.
He was a finalist for the school's most prestigious award given to students, Osborne said.
Osborne recalled Valdez being incredibly vigilant about pushing environmental changes at the school. For example, Osborne said, Valdez refused to give up when told his idea to have recycling bins at the school would violate the school's contract with its garbage collector.
The company collecting the school's trash separates the garbage from recyclables at its facility. Valdez felt that was doing nothing to teach students about the value of recycling, and so he pushed for bins at the school.
"He came back over and over again with a new idea on how to make it happen," Osborne said. "We ended up doing some smaller-scale recycling on campus."
Valdez also reportedly introduced "Lights Out Wednesdays" to the school, where an effort was made to shut off the lights in every unused classroom. He would calculate how much energy and money was saved on those Wednesdays, Osborne said.
Vigils for Valdez will be held at the high school's pool deck on Friday and Sunday.