The California Division of Occupational Safety and Health fined Tesla $2,700 last year for a serious violation at the Fremont factory that was uncovered during an accident investigation.
Agency spokesman Peter Melton said investigators found a hydraulic power press did not have a proper stop control. At least one person was injured, but Melton did not know the extent of the injuries.
Tesla was also fined for an accident at a different area facility two years ago in which a worker lost a fingertip.
Two of the three Tesla Motors workers who were burned when a machine malfunctioned at the Fremont factory remained hospitalized on Thursday.
One employee was seriously hurt and two others sustained minor injuries on Wednesday when a low-pressure aluminum casting press at the Fremont plant spilled hot metal, said Greg Siggins, a spokesman for Cal-OSHA.
"Hot metal somehow burned the workers," Siggins said.
The most seriously injured worker sustained upper-body burns, Melton said.
All three workers were taken to Santa Clara Valley Medical Center in San Jose.
Hospital spokeswoman Joy Alexiou said two remained hospitalized and one was released Wednesday. She declined to disclose their identities or conditions due to privacy laws.
Tesla CEO Elon Musk visited the workers on Wednesday at the hospital.
The incident was being treated as an industrial accident, Siggins said.
"We will be talking to any witnesses, reviewing training documents and manuals to see if they are in accordance with specifications as part of finding out what happened," Siggins said.
Tesla Motors, headquartered in Palo Alto, makes its all-electric Model S sedan at the Fremont factory that was previously owned by General Motors and Toyota.
Tesla took ownership of the plant four years ago and has transformed a portion of the cavernous site into a state-of-the-art facility.
The latest accident followed recent Tesla car fires in Washington state, Tennessee and Mexico that made headlines. Two Model S sedans caught fire after hitting a metal object in the road and a third caught fire after a high-speed chase.
None of the drivers was injured.
Musk has said there are no plans for a recall and insisted the Model S is one of the safest cars on the road.