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A University of California police officer, right, explains to an unidentified jogger that she cannot enter the campus after an explosion and fire in an underground utility vault caused a power outage and forced the evacuation, Monday, Sept. 30, 2013 in Berkeley, Calif. (D. Ross Cameron/Bay Area News Group)

BERKELEY -- Four people suffered minor burns and a mandatory evacuation was ordered on the UC Berkeley campus Monday after an explosion and a campuswide power outage that may be related to thieves who ripped valuable copper wire out of the university's electrical system, officials said.

The outage was first reported at 4:45 p.m., officials said. Around 6:30 p.m., as workers tried to fix the outage, an explosion was reported in an electrical vault outside California Hall in the heart of campus.

The explosion occurred as crews attempted to turn the power back on, said campus spokesman Dan Mogulof. The explosion was about two stories high and about as wide as a two lane street, he added.

Darkened stairs lead the way from Bancroft Avenue to the University of California after an explosion and fire in an underground utility vault caused a
Darkened stairs lead the way from Bancroft Avenue to the University of California after an explosion and fire in an underground utility vault caused a power outage and forced the evacuation of the campus, Monday, Sept. 30, 2013 in Berkeley, Calif. (D. Ross Cameron/Bay Area News Group)

"It was a fireball," said Christine Shaff, communications director for facility services, who was near the explosion when it happened.

Of the four people who were injured, three were treated at the scene and one was taken to a hospital.

About 20 people were trapped in campus elevators, but as of 8:30 p.m. all were free, Mogulof said. Campus laboratories were also quickly shut down and evacuated, including Latimer Hall, where there was a report of ammonia leaking, said Avery Webb, acting deputy chief for Berkeley fire.

Campus officials began to take steps to restore power about 9 p.m., with crews from Berkeley fire and Berkeley and campus police present at each of 14 electrical vaults on campus in case of a second explosion. The process may take as long as an hour and a half.

"We're concerned if we bring power up there might be another explosion," Mogulof said. "So we 're going to take it careful, we're going to take it slow and see what happens. We have reason to believe it will be successful but we're not going to take any chances.

"If that is successful, we expect campus to be open for regular business tomorrow, and if it's not, we're going to have to go back to the drawing board in terms of what happened to our electrical system."

All ongoing campus research dependent on electricity is being supported by off-site generators, Mogulof said. Generators are also providing power to off-campus residential halls.

The incident may have been caused by an attempted copper theft in underground facilities near the Lawrence Hall of Science in the hills area of the campus, officials said.

Last week, campus officials discovered that someone had used heavy equipment to pull the copper out of the ground. The damage was repaired Sunday, Mogulof said, but it appears the damage was more severe than originally thought.

Throughout the Bay Area, copper thefts have risen along with the price of copper, which can be sold for $4 a pound at salvage yards, nearly three times the price four years ago. The city of Concord has spent more than $1 million since 2007 on repairs and replacement. The city has gone as far as burying its copper wiring underground and placing boulders on top of the precious metal to prevent thefts.

David DeBolt covers breaking news. Contact him in Richmond at 510-262-2728. Follow him at Twitter.com/daviddebolt.