SANTA ANA ( — An explosion in a boiler room caused major damage to the gymnasium of an Orange County high school Tuesday, just minutes before classes were scheduled to begin, authorities said.

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The explosion occurred around 7:40 a.m. at Valley High School, 1801 S. Greenville Street, officials said.

“When units arrived on scene, they saw a large amount of smoke, but no flames,” Cpt. John Muir of the Orange County Fire Authority said.

The explosion caused the doors to the gym to be blown off, CBS2’s Michele Gile reported.

“It was just shrapnel all over the front of the area . . . lots of structural damage to the building. This was very significant,” Muir said.

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Seven students suffered minor injuries in the blast with four transported to Coastal Community Hospital as a precaution.

The campus was evacuated prior to the start of the school day as a result of the explosion.

It was unclear what caused the blast.

“Right now they are kinda narrowing it down to maybe a mechanical issue with regard to the boiler,” Muir said.

The bomb squad has been called into investigate. Foul play was not a factor in the blast, he said.

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Students were allowed to return to their classrooms after 8:30 a.m.