U.S. Navy officials said Tuesday that they have traced the 55 wooden ammunition crates, which washed onto beaches from Dana Point to Los Angeles last weekend, to the USS Comstock 30 miles offshore.
The crates, carrying only the packing tubes for 81-millimeter mortar rounds but no ammunition, were "incorrectly disposed of" at sea during a routine training exercise on June 25, said Lt. Karl Johnson of the Long Beach Naval Station.
The U.S. Coast Guard recovered 55 of them and is working with the Navy to find the five others, he said.
While the crates are not dangerous, "Navy regulations require compliance with environmental laws for any dumping of such material," Johnson said.
The Navy is investigating why the crates were dropped off the USS Comstock, which is based in San Diego, he said.
The Coast Guard called the Navy Sunday after boaters first spotted the crates, which are six inches deep, 15 inches wide and 24 inches long.