Military helicopters and ships scoured the waters off San Diego on Friday morning for any trace of nine people involved in a mid-air collision between a Coast Guard plane and a military helicopter hours earlier.

But after nearly 12 hours of searching, rescue teams had reported no signs of any survivors — only a field of debris floating in the Pacific. Military officials said they did not know how the crash had occurred, or which aircraft might have struck the other, saying they were still focused on search efforts.

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