UNIVERSAL CITY, Calif. — Fire-damaged Universal Studios reopened Monday as 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 park, which was closed Sunday after the early morning fire.

On the first tram tour of the day, the guide mentioned the fire and thanked firefighters for their work. Visitors could smell smoke and see the fire damage minutes into the tour.

The fire, which began early Sunday, 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.

Universal Studios is a theme park and its back lot is 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. The park was to reopen Monday.

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 morning.

The fire, the second at the historic site in two decades, leveled facades, creating the kind of catastrophe filmmakers relish re-creating. Thousands of videos from Universal's movie and TV shows were destroyed.

But Universal officials said that 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 NBC Universal President and Chief Operating Officer Ron Meyer. "Nothing is lost forever."

Two mock New York and New England streets used both 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.

Hundreds of visitors who waited for hours outside the park gates were turned away Sunday. On a typical weekend day, about 25,000 people visit Universal Studios.

An adjacent shopping promenade also was closed. However, the MTV Movie Awards was broadcast live as planned Sunday night from the nearby Gibson Amphitheater.

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