Jade Harris, 30, of Los Angeles, went to the Downey office to inquire about a Chevy Camaro advertised on Craigslist and for some reason allegedly pulled a gun and opened fire.
"We are trying to make sense of these events," Downey police Lt. Dean Milligan told the Los Angeles Times.
Three people died and two were injured in the shooting, which actually took place at two locations.
Harris is charged with shooting three people inside the business and then forcing another employee and her 13-year-son to drive him the nearby home where the Camaro was parked. He then allegedly shot and killed the woman and wounded the boy, who played dead until Harris left the scene.
The Times said Downey police had not stated any reason why the incident deteriorated into violence so quickly. They noted Harris, who declined to enter a plea in court Monday, was an unemployed parolee and member of an LA street gang.