The man accused of killing seven fellow farm workers and wounding another in California could face the death penalty if convicted of multiple murder charges, a prosecutor said, as the suspect appeared in court for the first time.
Zhao Chunli, 66, was arrested on Monday in Half Moon Bay, south of San Francisco, shortly after twin attacks at farms that left five men and two women dead.
Mr Zhao appeared in court in nearby Redwood City on Wednesday wearing jail clothing.
He faces seven counts of murder, one of attempted murder and what prosecutors called a special circumstance allegation of multiple murder.
A Mandarin language translator was on hand for a hearing in which his attorney requested the formal arraignment be postponed.
San Mateo County Superior Court Judge Susan Jakubowski remanded Mr Zhao in custody and ordered that he appear in court on 16 February.
San Mateo County District Attorney Stephen Wagstaffe told the San Francisco Chronicle that the special circumstance allegation "makes him eligible for the punishment of life without parole or death".
"That decision will be made down the road after the full investigation is completed and we learn everything we can about Mr Zhao," Mr Wagstaffe added in an email to the paper.
San Mateo County Sheriff Christina Corpus said earlier this week that it appeared the suspect had known the victims.
"All the evidence we have points to this being an instance of workplace violence," she said.
Mr Zhao is believed to have used a legally purchased semi-automatic pistol in the rampage.
The suspect is a Chinese national who has lived in the Half Moon Bay area for at least a decade, including for seven years at the mushroom farm where the killing spree began, the San Jose Mercury reported.
The paper cited Captain Eamonn Allen saying the sheriff's department was working to support the suspect's wife.
"They lived on the property together, so there's a very real concern of retribution or backlash on her," Mr Allen said.
Charging documents and information from the coroner reveal five of the dead had Chinese names, with two of them being aged in their 70s.
There was no immediate reply to AFP queries to Chinese consulates in San Francisco and Los Angeles.
Mexico's consulate in San Francisco confirmed two of the dead were Mexican men. Another Mexican man survived the attack and is in hospital in Palo Alto, the consulate said.
The Half Moon Bay attacks came on the heels of another deadly rampage in California on Saturday in which an elderly Asian gunman killed 11 people at a dance club in Monterey Park, near Los Angeles.
He shot himself dead the following morning as police moved in.
The two horrific episodes, both involving semi-automatic weapons, sparked bafflement from California's large Asian American community, as people struggled to come to terms with what had happened.
They also led to outrage over America's lax attitude to gun control, with California Governor Gavin Newsom blasting federal inaction.
"What the hell is wrong with us, that we allow these weapons of war and large capacity clips out on the streets and sidewalks?" he said.
"Only in America. Number one in gun ownership. Number one in gun deaths. It's not even complicated," he said.