In the very early morning hours of February 25, 1942, Angelenos were jolted out of bed by screaming air raid sirens and thundering anti-aircraft gunfire. Hundreds of thousands of residents, ignoring blackout rules, snapped on lights and spilled into the streets to watch sweeping searchlights and orange tracer shells streaming into the night sky. Thousands of volunteer air raid wardens grabbed helmets and boots to rush into action. Frightened drivers, speeding through darkened streets, collided with one another, resulting in three traffic fatalities. As many as three other persons were reported to have died from fatal heart attacks. Gunners expended more than 1,400 rounds of ammunition that caused quite a bit of damage to structures on the ground. Many Angelenos believed that they were going to see a Japanese invasion force lying offshore when daylight broke. Although explanations for what had spooked military gunners ranged from a false alarm to meteorological balloons to UFOs, an investigation could not clearly determine the exact cause. The incident turned out to be the only serious military action to occur over a continental U.S. metropolitan area during World War II.