From the article and news video:
Police in Antioch believe three or four male partygoers opened fire Saturday night when two groups began arguing, said police Lt. Scott Willerford. Violence is not common in the neighborhood in Antioch, which is about 38 miles northeast of San Francisco.
"Weapons were brandished and numerous shots were fired from multiple guns," Willerford said.
The gunfire wounded a 13-year-old boy, two 16-year-old boys, two 18-year-old men and a 21-year-old woman, police said. Their injuries ranged from a minor grazing wound to an abdomen wound, police said. Police did not say how many shots were fired, or specify what kind of weapons were used.
Three of the wounded have been released from hospitals, but the 13-year-old, an 18-year-old and the woman remained in serious but stable condition, police said. Their names have not been released.
Every gun in the hands of a child must first pass through the hands of an adult.