LOS ANGELES – The dramatic Sunday blaze that destroyed portions of Universal Studios Hollywood back lot was accidentally started in predawn hours by company employees doing roof work with a blow torch, authorities said.

Los Angeles County fire officials said two workers and a supervisor were using the torch to heat roofing shingles on the “New York Alley” set. They finished at 3 a.m. PDT, watched for any fire starts for an hour and then took a break.

At 4:43, the same time the crew returned, a security guard spotted flames and reported the fire.

The studio’s theme park and City Walk, the adjacent business district, reopened Monday as the Los Angeles County Fire Department launched what County Fire Chief P. Michael Freeman promised would be a “stem to stern” examination of the water pressure problems that hampered the three-alarm attack on the blaze that destroyed back lot sets, a video library and the “King Kong” exhibit.

The fire, which burned about two city blocks, was not extinguished until about 10 p.m. PDT Sunday. Monday morning, 40 to 50 firefighters were still dousing hot spots.

The fire affected about 3 1/2 acres of the 391-acre park, Universal said, with estimated damages “in the millions” of dollars.