In late February 1942, an unknown object was seen in the sky above LA. The day before, a Japanese submarine had bombed the American oil field of Ellwood so the Americans were on edge.
Immediately, the sky was lit up by search lights and the anti-aircraft guns were fired almost at once. Whatever was in the sky was unharmed, but several buildings in LA were destroyed by the anti-aircraft guns, and six American civilians were killed, plus many more injured.
To this day it's unclear exactly what was seen above the city that day, but it quickly became clear that it was not hostile, and 'all clear' was declared at 4:14 in the morning, the panic was passed off as a case of 'war nerves'.