Officials said the courthouse square and the clock tower that enabled Michael J Fox's character to time travel in Back to the Future were damaged.
The set from the current hit television series Desperate Housewives was destroyed as were sets from films such as The Sting, The War of the Worlds, Ben-Hur and Psycho.
Daryl Jacobs, the Los Angeles county fire inspector who announced the preliminary investigation results, said at least one building had burned and as many as three blocks of film facades were destroyed.
He did not reveal the specific cause of the fire.
"The facades are constructed of heavy timber and they tend to burn quite freely," he said.
The video vault contained tens of thousands of videos and movie reels before it was destroyed, but Ron Meyer, the Universal Studios chief executive, said its main archive did not suffer irreversible losses.
"The video library was affected and damaged, but our main vault with the motions pictures negatives was not," he said.
"Nothing irreplaceable is lost... We were very lucky. It's a bad situation, but it could have been a lot worse."