(NewsNation) — A farmworker accused of killing seven people in back-to-back shootings at two Northern California mushroom farms was charged Wednesday with seven counts of murder and one of attempted murder.

Chunli Zhao, 66, was set to make his first court appearance Wednesday but it was postponed until Feb. 16, San Mateo County District Attorney Steve Wagstaffe said. Zhao’s two attorneys did not immediately respond to calls and emails seeking comment.

The incidents were described as workplace violence by police, who have not offered any additional details on the motive behind the shootings. They arrested Zhao in the parking lot of a sheriff’s office substation after the shootings.

Authorities believe Zhao acted alone Monday when he entered a mushroom farm where he worked in Half Moon Bay and opened fire, killing four people and seriously wounding a fifth, San Mateo County sheriff’s officials said. He then drove to a nearby farm where he had previously worked and killed an additional three people, said Eamonn Allen, a sheriff’s spokesperson.

Law enforcement officials have not specified if Zhao has a criminal history, but the San Francisco Chronicle reported that there had previously been a restraining order against him after he attacked a roommate and co-worker. In 2013, Zhao was accused of threatening to split a co-worker’s head open with a knife and separately tried to suffocate the man with a pillow, the Chronicle reported, based on court documents.

The coroner’s office named six of the victims: Zhishen Liu, 73, of San Francisco; Marciano Martinez Jimenez, 50, of Moss Beach, California; Aixiang Zhang, 74, of San Francisco; Qizhong Cheng, 66, of Half Moon Bay; Jingzhi Lu, 64, of Half Moon Bay; and Yetao Bing, 43, whose hometown was unknown. The charging documents identify Jose Romero Perez as the other person killed and Pedro Romero Perez as the eighth victim, who survived the shooting.

Servando Martinez Jimenez said his brother Marciano Martinez Jimenez was a delivery person and manager at one of the farms. Servando Martinez Jimenez said his brother never mentioned Zhao or said anything about problems with other workers.

“He was a good person,” he said in Spanish of his brother. “He was polite and friendly with everyone. He never had any problems with anyone. I don’t understand why all this happened.”

Marciano Martinez Jimenez, 50, had lived in the United States for 28 years after arriving from the Mexican state of Oaxaca. Servando Martinez Jimenez said he is working with the Mexican consulate to get his brother’s body home.

KTVU also reported that a family member identified Jose Perez as one of the casualties and his younger brother Pedro Perez as the only survivor. Jose Perez left a family behind in Mexico.

The shootings in Half Moon Bay came less than 48 hours after the deaths of 11 people on a Southern California dance floor at the Star Dance Studio in Monterey Park.

Kristenne Reidy, the daughter of 68-year-old Valentino Alvero, said her father loved the Star Dance Studio, where his van is still parked out front.

“I like to think that he danced his way into heaven,” Reidy said. “I’m standing by his van, I can’t believe it’s still here.”

While the exact motive of the gunman who died by suicide may never be known, members of the Asian community say 72-year-old Huu Can Tran had a temper. They also say he was not invited to the Chinese New Year countdown dance party that was being held the night of the shooting.

According to a new study by the National Threat Assessment Center, 75% of deadly shootings involve personal grievances.

“These were attackers who were retaliating for some sort of perceived wrongs that may have been related to either personal issues, domestic situations with partners as well as workplace issues,” said Lina Alathari, the center’s chief.

The new federal research notes that attacks are overwhelmingly carried out by men with a history of domestic violence or mental health issues, and guns are usually the weapon of choice.

California is among the 19 states with a red-flag law, which allows the removal of guns from potentially dangerous individuals. Another vigil is planned for Wednesday night in Monterey Park along with a visit by Vice President Kamala Harris.

The Associated Press contributed to this report.