A loud boom on Wednesday generated calls to emergency agencies in Orange County, but authorities had trouble tracking down what caused it -- until the U.S. Navy confessed to being the culprit.
An earthquake was quickly ruled out as the cause.
"We reached out to Caltech, and they said it's not seismic; it's sonic-related,'' said Vicki Osborn, an assistant emergency manager in the sheriff's department.
Officials at the Camp Pendleton Marine base were also mystified.
The base, south of the Orange County line, had no training scheduled or conducted today, Sgt. Christopher Duncan said.
"Right now we're calling around, seeing if any aviation assets maybe created some sound,'' Duncan said this afternoon.
An official at Edwards Air Force Base north of Lancaster was also unaware of any sonic events.
By late afternoon, however, it appeared the U.S. Navy was to blame for the commotion.
The Navy confirmed to the Los Angeles Times that it had conducted an exercise about 50 miles off the coast, with an aircraft flying faster than the speed of sound, causing a sonic boom.