Fire officials determined the blaze was caused by accident.
The smell of smoke hung in the air as guests streamed in when the gates reopened and tourists on the park's tram ride applauded firefighters as they drove past. At least a dozen fire trucks remained on the lot as smoke continued to rise from thick, twisted piles of girders.
"We were a bit shocked," said Danish tourist Morten Jull, 20. "We were like, can this be?"
The pass by the fire scene was brief. The tram also drove by undamaged sights, including sound stages, the "Jurassic Park" area where dinosaurs spray water at visitors, the Bates Motel from "Psycho" and Wisteria Lane, scene of TV's "Desperate Housewives."
Later, on a viewing platform overlooking the back lot, several people surveyed the destruction.
"It's sad for Hollywood,' said Ismael Garza, 53, a stage manager.
A preliminary investigation determined the fire was an accident, said Los Angeles County fire Capt. Mike Brown. He would not provide further details, but county officials planned a Monday afternoon press conference.
The fire erupted about 4:30 am on Sunday (US time) on a streetscape featuring New York brownstone facades at the 400-acre (160-hectare) property. It then destroyed a King Kong attraction, the courthouse square from Back to the Future and a streetscape featured in Spider-Man 2 and Transformers.
The blaze also gutted a building housing thousands of videos, but Universal Studios President and Chief Operating Officer Ron Meyer said there were duplicates of everything.
Low water pressure forced firefighters to tap lakes and ponds at Universal, which is a working studio as well as a theme park.
The blaze burned for more than 12 hours but was contained to the back lot, which straddles a pass through the hills between Hollywood and the San Fernando Valley.
Universal Studios said it could take several days to assess damage, but it was "business as usual" at its theme park and TV and movie production resumed Monday.
"The studio is open, production has resumed and our theme park and CityWalk reopened for business today," said spokeswoman Cindy Gardner in a statement.
None of the 30 sound stages on the lot were damaged, and the New York streetscape will be rebuilt, she said.
Nine firefighters and a sheriff's deputy suffered minor injuries. The deputy and a firefighter were injured in an explosion in the building where the videos were housed, authorities said.
Art director Francois Audouy helped shoot the final battle scene of "Transformers" on the New York streetscape, and said it was the longest and widest of its kind in southern California.
"It's a real shame," Audouy said. "There's a New York street at Warner Bros., and Paramount, as well. But Universal's was considered a very unique option for filmmakers to create that New York look."
The set was held up by telephone poles and wood, and a fire crew was standing by at all times over the four days of shooting, which included many explosions, he said.
Audouy said he hoped Universal would rebuild the set with a less-flammable steel skeleton.
"It's too big of an asset for Hollywood. It's too great of a resource," he said, noting production companies paid tens of thousands of dollars a day to rent the space. "Plus, it's one of the highlights of the Universal back lot tour for tourists."
Los Angeles County Fire Chief Michael Freeman on Sunday called the water pressure situation "a challenge." A sprinkler system on outdoor sets was nearly useless, he told the Los Angeles Times.
Universal is in county territory and operates and maintains its own water system, which is fed by the Los Angeles Department of Water and Power. But the city's involvement stops at Universal's property line.
"Yesterday we had adequate water supply feeding their system and were in fact asked by the Fire Department to attempt to increase water pressure and we did so," DWP spokesman Joe Ramallo said Monday.
"However that action had a negligible effect because of the area in which Universal is located and the fact that the water system that runs throughout Universal Studios is private and operated by the park," he said.
Another fire at Universal Studios in November 1990 caused $25 million in damage and was started by a security guard who was sentenced to four years in prison after pleading guilty to arson. After that fire the New York streetscape was rebuilt in three months.
Sunday's fire sent an acrid plume of smoke into valley neighborhoods to the north. Results of air samples taken by the South Coast Air Quality Management District were expected sometime Monday.
The destroyed streetscape had recently served as a backdrop in television shows like "Monk," ''Crossing Jordan" and "House," said NBC Universal spokeswoman Cindy Gardner.
A set used for the Clint Eastwood-directed movie "Changeling" featuring Angelina Jolie also was destroyed, Meyer said.
Meyer estimated there were 40,000 to 50,000 videos and reels in a video vault that burned but said duplicates were stored in a different location. Firefighters managed to recover hundreds of titles.
The videos included every film that Universal has produced and footage from television series including "Miami Vice" and "I Love Lucy."
