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SAN DIEGO – A loud boom echoed through a Southern California county late Tuesday, but its cause remains a mystery, officials said.

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According to KSWB and KGTV, the incident occurred shortly before 8:30 p.m. local time in San Diego County. Residents, including San Diego Mayor Todd Gloria, flocked to social media to say they heard the noise and felt shaking, the outlets reported.

“Yes, I heard it. No, I don’t know what it was. I’ll share if I get any information. Get vaccinated if you haven’t yet,” Gloria tweeted, adding the hashtag #SanDiegoBoom.

Although some speculated that an earthquake had struck the area, the U.S. Geological Survey did not report one at the time, according to the news outlets.

KSWB and KGTV later reported that training exercises at nearby Camp Pendleton may have caused the sound.

“This week, MCB Camp Pendleton is firing high explosive munitions (mortar and artillery fire) within our training ranges, which may take place between 6 a.m. to midnight,” the base said in a statement to KGTV. “What you hear may be the result of this training.”

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