Investigators concluded on Monday that the blaze at Universal Studios' back lot was sparked by studio workers who were repairing a New York street scape.
Meanwhile, the theme park reopened after the massive fire forced it to close on Sunday.
The fire, which started Sunday morning, was ignited by repairing tools used by workers who are veteran Universal employees, the Los Angeles Times said, quoting sources close to the investigation.
It appeared the workers followed all protocols to prevent fire, the report said.
Los Angeles County Fire Chief P. Michael Freeman said earlier that the fast-moving fire was an accident.
"The investigation has ruled out any deliberate actions on the part of anyone," Freeman said.
Studio spokesman Eliot Sekular said no filming was going on when the fire broke out.
Nine firefighters and a sheriff's deputy suffered minor injuries in the towering inferno which burned for more than 12 hours.
The flames destroyed huge chunks of movie history including sound stages and much of the New York City streetscape used for exterior shots. It also burned through the building that housed a mechanical King Kong that delighted tourists on the popular backlot ride, and gutted a video warehouse.
At the large warehouse, videos of films going back to the 1920s were destroyed, but everything inside the video vault was a copy of material stored elsewhere, and "nothing precious was lost" said Ron Meyer, president of Universal Studios.
Film negatives and other priceless materials were stored in underground buildings several hundred yards from the fire, he said.
(Xinhua News Agency June 3, 2008)