Two brothers suffered serious stab wounds in the chest late Saturday during a fight in the parking lot of Irvine Meadows Amphitheatre.
Police said the fight erupted between two groups shortly before midnight, after a concert by the metal band Anthrax and the Public Enemy rap group.
"An argument started and someone in one group accused someone in the other of the theft of some jewelry from one of the cars," Police Sgt. Tom Little said. "They were yelling back and forth for about 10 minutes," then someone asked a man passing by for help.
The passer-by "took over and became the focal point" of the dispute, using a knife to flatten a tire of a vehicle belonging to one of the parties, Little said.
The brothers, both from Moreno Valley, tried to stop the man, and each was stabbed in the chest. The assailant then fled into the crowd.
Curtis Barger, 19, was listed in stable condition with a slashed liver Sunday night at Mission Hospital Regional Medical Center in Mission Viejo, while his brother, Wayne, 24, was in serious condition.
The suspect was described as white or possibly Latino, in his middle 20s, about 5 feet, 10 inches and 230 pounds, with dark curly hair and a mustache.