UNIVERSAL CITY, Calif. — A massive blaze that destroyed part of the back lot at Universal Studios was accidentally ignited by workers using a blowtorch on the roof of a movie set building facade, fire officials said.

Workers had been using the blowtorch early Sunday to heat asphalt shingles to apply to the roof, Los Angeles County Fire Chief Michael Freeman said Monday. They finished around 3 a.m. and followed policy of standing watch for one hour, then left for a break, he said.

A security guard spotted the fire and reported it at 4:43 a.m., Freeman said.

The fire erupted on a streetscape featuring New York brownstone facades at the 400-acre 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 theme park reopened Monday, giving tourists a view of the fake streetscape that went up in real flames, and authorities promised to investigate reports that firefighters were hindered by low water pressure.

Freeman suggested the problem may have been due to the volume of water — 18,000 gallons a minute at the peak — that was poured on the flames. He said the studio's water systems were up to current county code.

The low water pressure forced firefighters to tap lakes and ponds at Universal, which is a working studio as well as a theme park straddling a pass through the hills between Hollywood and the San Fernando Valley.

It lies 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.

"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.

The blaze burned for more than 12 hours but was contained to the back lot. It gutted a building housing 40,000 to 50,000 videos, but Universal Studios President and Chief Operating Officer Ron Meyer said there were duplicates of everything.