An immigrant farm worker accused of shooting dead seven people near San Francisco, some of them his co-workers, made his first court appearance on Wednesday after he was charged with murder.
It was California's second deadly gun rampage in recent days.
Chunli Zhao, 66, the lone suspect in Monday's massacre at two mushroom farms in the seaside town of Half Moon Bay, was formally charged with seven counts of premeditated murder and one of attempted murder.
The next court proceeding in the case was set for February 16.
The prosecutor said authorities did have an idea about the suspect's motives but declined to share it.
The district attorney said Mr Zhao was "co-operative with sheriff's detectives" who initially interviewed him through a Mandarin interpreter, without an attorney present, after his arres.
They said he gave "a complete statement."
In addition to eight felony counts, the 10-page criminal complaint alleges "special circumstances", accusing Mr Zhao of "personally and intentionally" shooting to kill.
The Half Moon Bay killings unfolded two days after another gunman, 610km to the south, opened fire at the Star Ballroom Dance Studio, a club frequented mostly by older patrons of Asian descent in Monterey Park.
Eleven people died and nine were injured in Saturday night's gunfire, which some survivors and bystanders said they initially mistook for fireworks as the largely Asian-American community was observing the start of the Lunar New Year.
Authorities said the assailant, Huu Can Tran, 72, shot himself to death behind the wheel of his getaway vehicle as police closed in on him south of Los Angeles.
That left investigators with few clues as to what led to the dance hall carnage.
US Vice President Kamala Harris, a California native,, planned to visit the Los Angeles suburb of Monterey Park, site of the first of the recent deadly rampages, and meet with some families of the 11 people who were killed on Saturday.
Coming in quick succession, the two shooting sprees left California reeling from one of the deadliest spates of mass gun violence in decades.
The state is promoted by its Democratic political leadership for some of the strictest firearm laws in the country.
Agencies contributed to this report