Thick black smoke that billowed into the Los Angeles sky was partly due to the plastic video containers burning, said Los Angeles fire department chief Michael Freeman.
Low water pressure hampered the fire crews' efforts, according to the LA Times.
The head of Universal Studios, Ron Meyer, said about 40,000 to 50,000 videos and reels had been damaged in the blaze but duplicates were kept elsewhere.
He thanked the firefighters for their efforts.
"They are real heroes, I mean they were here in record time," he said.
"They contained the fire that could've really burnt horribly out of control."
Behind-the-scenes tours of the studios are a long-established tradition still popular with visitors today.
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