The incident happened Sunday night inside a warehouse the show was being held in, KTLA-TV, Los Angeles, reported.
The victim, identified by family members as Mia Chapman, was struck by an object, her family said.
"We're getting multiple stories, so that's another issue," said Stacy Young, a cousin of Mia's. "I've heard that it was water that fell. I've heard that she was hit by a car, but ... nobody's telling us anything."
Police at the scene declined to comment on the investigation, but said no one has been arrested in the case and that no charges are pending.