For the next two hours Mulholland, Van Norman and Harnischfeger inspected the dam and various leaks and seepages, finding nothing out of the ordinary or of concern for a large dam. With both Mulholland and Van Norman convinced that the new leak was not dangerous and that the dam was safe, they returned to Los Angeles.
Two and a half minutes before midnight on March 12, 1928, the St. Francis Dam catastrophically failed. The main dam, from west of the center section to the wing wall abutment atop the hillside, broke into numerous smaller and several large pieces. All of these were washed downstream as the 12.4 billion gallons (47 billion liters) of water contained in the reservoir began surging down San Francisquito Canyon. The largest piece, weighing 10,000 tons, (9,100,000 kg) was found about three-quarters of a mile (1200 m) below the dam site.
To this day, the exact number of victims remains unknown. The official death toll in August 1928 was 385, but the remains of victims continued to be discovered every few years until the mid-1950s. Many victims were swept out to sea when the flood reached the Pacific Ocean and were never recovered while others were washed ashore, some as far south as the Mexican border. The remains of a victim were found deep underground near Newhall in 1992 and another believed a victim of the disaster was found in 1994. The current death toll is estimated to be more than 600 victims, excluding the itinerant farm workers camped in San Francisquito Canyon, whose exact number will never be known.