Tourists applauded firefighters Monday as Universal Studios reopened, while investigators examined the ruins of some of the most famous sets in Hollywood to find the cause of the spectacular weekend blaze.

When the gates opened, hundreds of people streamed into the venerable movie studio-theme park, which was closed Sunday after the early morning fire.

Tourists on the tram ride through the sets gave a round of applause to firefighters still putting out hot spots. At least a dozen fire trucks remained on the lot and smoke was still rising from thick, twisted piles of girders.

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The fire tore through the back lot as firefighters struggled to overcome low water pressure and an overwhelmed sprinkler system.

"The water pressure situation was a challenge," Los Angeles County Fire Chief Michael Freeman said. "This fire moved extremely fast."

County Supervisor Zev Yaroslavsky said authorities would investigate the water problems to see if they reflect a larger shortfall in the area.

"There's no question that there was a lack of adequate water pressure at least in the perception of a lot of firefighters," he said. "We're going to find out what the problem was."

In addition, the sprinkler system on the outdoor sets was nearly useless, Freeman told the Los Angeles Times for Monday's editions.

The cause of the blaze had not yet been determined.

While 25,000 tourists visit on a typical weekend day, Universal Studios' back lot is also a working studio, with streetscapes and soundstages. The fire, which broke out around 4:30 a.m. Sunday, destroyed the courthouse square seen in "Back to the Future."

Damage estimates were not available, but costs are expected to rise into the millions.

A thick column of smoke rose thousands of feet into the air and could be seen for miles. "It looked like a disaster film," said Los Angeles City Councilman Tom LaBonge.

Concerns about air quality because of the acrid smoke and a request from fire officials prompted the South Coast Air Quality Management District to send a chemist to take air samples, said spokesman Sam Atwood. Results were expected Monday.

The fire, the second at the historic site in two decades, leveled facades, creating the kind of catastrophe filmmakers relish re-creating.

NBC Universal President and Chief Operating Officer Ron Meyer estimated there were 40,000 to 50,000 videos and reels of film in a vault that burned but said duplicates were stored in a different location. Firefighters managed to recover hundreds of titles.

Universal officials said they were thankful no visitors were seriously injured and that the damaged footage can be replaced. The videos included every film that Universal has produced and footage from television series including "Miami Vice" and "I Love Lucy."

"We have duplicates of everything," said Meyer. "Nothing is lost forever."

Two mock New York and New England streets used for movies such as "Bruce Almighty," "Spider-Man 2" and "Transformers" and as tourist displays were a total loss, Los Angeles County Fire Inspector Darryl Jacobs said.

The city streetscape has 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.

Along with the courthouse square, the famous clock tower that enabled Michael J. Fox's character in "Back to the Future" to travel through time was damaged, fire officials said.

Ten people — 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.

Universal City, nine miles north of downtown Los Angeles, was founded in 1915 by Carl Laemmle, the founder of Universal Studios. Laemmle created the first studio tour, which consisted in its earliest days of people standing on a platform or on bleachers while filming took place below.

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