Police arrested 22-year-old Law Thien Huynh of Gardena in connection with the killing at about 4 p.m. Wednesday. Huynh was taking Robert Herrera's security guard training class at Coast Career Institute, located on the 1300 block of South Hill Street.
Police say on the day of the shooting, Huynh got into an argument with Herrera, 44, and walked out of class briefly. He returned with the gun and opened fire.
"The guy was in class, and he left briefly and came back with a gun and kicked the door down and shot Mr. Herrera in the head once, and then four times in the chest," said Jean Roberts, a student who was in the classroom at the time. "Like he just -- his whole motive was to kill him."
Herrera was declared dead at the scene. No one else was injured in the incident.
After the shooting, witnesses said Huynh simply put the gun down and told his classmates to call 911.
When officers arrived on scene, Huynh was outside smoking a cigarette. He was taken into custody without incident, and a weapon was recovered at the scene.
Huynh has been charged with murder and was booked at the downtown Metropolitan Jail.
Flowers and candles covered the entrance to Coast Career Institute Thursday. The school was closed, but students gathered to mourn Herrera's death.
"Robert was a good instructor. He has love from all his students," said student Domonique Butler. "Everybody loved him. He treated us like family, like friends, not just students."
According to students, Huynh often asked questions about what it takes to shoot someone, and volunteered himself to be pepper-sprayed in the face as part of one demonstration.
Coast Career Institute was closed Thursday. It was scheduled to reopen Monday.