Authorities responded to the shooting at Edison's No. 3 building on the 4900 block of Rivergrade Road at about 1:30 p.m. Officials said the building houses the company's I.T. department, and there were about 200 employees in the building.
It was initially reported that there were no fatal shooting victims other than the gunman, but during a search of the building, a deceased person was found. Another shooting victim died en route to a hospital.
Two other people were rushed to local hospitals.
A man and a woman wounded by gunfire were listed in critical condition.
The victims were not identified.
"It's a very sad day for us," said Edison International Chairman Ted Craver.
The shooter was an employee who worked in the building. Three of the shooting victims were also employees. The fourth shooting victim was a contractor for the company. Three of the victims were men, the fourth a woman.
"We are in the process of contacting family members. We're also providing couseling services to all of our employees at the other facilities," said Ted Craver, chairman of Edison International.
Authorities evacuated the employees in the building, and a SWAT team went room to room to ensure there were no other suspects or victims. Dozens of people were seen streaming out of the building with their hands raised.
Monica Salazar said her mother, a 30-year Edison employee, was inside the building at the time of the shooting. Her mother communicated through text message, saying a group of them had locked themselves in a conference room.
As Salazar was being interviewed by Eyewitness News, she was reunited with her mother.
"We were just told to lock ourselves in a room, and that was it, so that's all we did," she said.
Olive Middle School Walnut Elementary School in the Baldwin Park Unified School District locked down, but the lockdown was later lifted.