Dozens of people were injured when a wood platform holding live fireworks tipped over, sending the pyrotechnics into a crowd at an Independence Day show outside Los Angeles.
Between 8,000 and 10,000 revellers were taking their seats for the fireworks extravaganza at a park in Simi Valley when the spectacle of lights that were supposed to be launched in the sky never took off.
Instead, a bright plume of red and white bursts spread across the ground, sending onlookers fleeing for safety.
Thirty-nine people were injured. The victims, aged from 17 months to 78, had burns and shrapnel wounds, and some were trampled, authorities and hospital officials said. The casualties included 12 children.
Only three remained in hospital early today.
Police were still investigating what caused the explosion, but they said it appeared a firework detonated prematurely in its mortar, knocking over a row of others.
The mortars toppled like "dominoes" and one or more fired into a crowd of spectators some 800 feet away, police commander Blair Summey told the Los Angeles Times.
"For some unknown reasons the structure that holds these ordinances collapsed and caused them to be firing into the crowd," said Simi Valley police commander Stephanie Shannon.
Four people were in a serious condition, but their injuries were not considered life-threatening. Sixteen more were taken to hospitals with minor to moderate injuries. The remainder were treated at the park where emergency crews, already on hand as a safety precaution, set up a triage area.
Meanwhile, a worker at a fireworks show in North Myrtle Beach, South Carolina, was injured on Thursday night when a shell exploded prematurely. The explosion at the Cherry Grove Pier caused the show to end early, after just six minutes, and left a hole in the pier.
In California, a video clip aired on KCAL-TV shows a pair of firework blasts at or near the ground. Another clip, posted on YouTube and shot from a distance, shows three ground-level bursts. The fireworks continue for almost another minute before stopping.
"There was a big boom, everybody started running down the street, people were screaming," Justice Allen, 17, of Simi Valley told the Los Angeles Times. "Everybody was just terrified. People hid in bushes."
Authorities estimate people were 900 feet away from where the fireworks were being launched. One police officer who ran into the crowd when the blasts occurred had shrapnel tear through his leather belt and his clothing, Ms Shannon said. He had minor injuries to his back.
The annual July 4 celebration has been sponsored by the city and the local Rotary Club for the past 43 years. Ms Shannon said she had no information about the company hired for the fireworks show.
Simi Valley, home to the Ronald Reagan Presidential Library, is about 40 miles north west of Los Angeles.