The 66-year-old former farmworker is accused of killing seven people in back-to-back shootings in California last month.
The 66-year-old suspect, Chunli Zhao, entered the plea himself on Thursday, speaking from behind a glass partition with his head lowered. He used a Mandarin-language interpreter to communicate.
He faces seven counts of murder and one count of attempted murder for his alleged role in the mass shooting, which took place on January 23 at farms where he had previously been employed.
Zhao had previously admitted to carrying out the shooting in a jailhouse interview with local outlet KNTV-TV, saying he had been bullied and worked in difficult conditions that went ignored during his time on the farms.
San Mateo County Judge Elizabeth K Lee issued a gag order on Friday to prevent Zhao, his defence, the prosecution and the local sheriff’s office from speaking to reporters about facts or opinions regarding the ongoing case.
The shooting rampage, which took place 48km (30 miles) south of San Francisco, began at the Mountain Mushroom Farm where Zhao was employed at the time. He allegedly killed four co-workers, wounding a fifth.
He is then accused of driving to a second location, Concord Farms, where he had worked for about four years before being fired in 2015. There, Zhao is alleged to have shot and killed three former co-workers.
The attack — which took place days after another mass shooting in Monterey Park in southern California — drew attention to the persistent issue of gun violence in the US, with President Biden calling it a “senseless act”.
The Half Moon Bay shooting also underscored the harsh conditions faced by California’s poorly paid, largely migrant agricultural labour force. All of the victims in the shooting were of Asian or Latino descent.
In the wake of the shooting, California Governor Gavin Newsom and other officials visited victims’ families and toured Half Moon Bay, a coastal community known for tourism and agriculture.
“Some of you should see where these folks are living — the conditions they’re in, living in shipping containers,” Newsom said in a press conference on January 24. “Folks getting nine bucks an hour. You wanna verify the California minimum wage? It’s not $9 an hour. No healthcare, no support, no services.”