The avalanche happened about 10:15 a.m. and the resort shut down operations shortly thereafter, officials said.
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No one had been reported missing as of about 11:15 a.m., but a search was launched in accordance with protocol, officials said.
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According to Mammoth Mountain's Twitter page, three people were partially buried in the avalanche but were able to free themselves.
Local police were asking the public to steer clear of the area as emergency crews responded.
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Mammoth Mountain tweeted that avalanche mitigation work was being done on the upper part of the mountain when an avalanche released traveled toward ski areas.
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Resort representatives asked anyone with missing friends or family to call (760) 934-0611.