California ? Workers using a blowtorch on the roof of a movie set building at Universal Studios accidentally set the huge fire that swept through the back lot, fire officials said Monday.

Workers had been using a blowtorch to heat asphalt shingles to apply to the roof early Sunday, said Los Angeles County Fire Chief Michael Freeman. They finished about 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.”