The girl, identified by family members as Sasha Rodriguez, was one of two rave attendees who were in critical condition at California Hospital Medical Center after the 14th annual Electric Daisy Carnival.
As Sasha's family decided whether to remove her from life support Tuesday, her mother, Grace Rodriguez, told the CBS Evening News: "I was supposed to be planning her Sweet Sixteen party. Now I have to plan her funeral."
Sasha, who lived in the Atwater neighborhood, died shortly before 5:30 p.m. Tuesday, according Katreena Salgado, a spokeswoman for the hospital.
She and the other critically ill patient had been taken to the downtown Los Angeles hospital by ambulance directly from the venue, officials said. Both were treated for drug intoxication in the intensive care unit.
"She came in as an emergency patient from the rave. She was in respiratory arrest when she got here, and she never recovered," said Salgado, who said Sasha was in a coma and experienced multiple organ failure.
Outside California Hospital Tuesday night, friends and family said that when Sasha was found, she did not have identification.
"Obviously they didn't check IDs," said Eva Rodriguez, Sasha's godmother.
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Doctors told Sasha's family that she had the drug ecstasy in her system when the ambulance got her to the emergency room.
Kimberly Keith, a family friend, said Sasha's loved ones are still trying to figure out what happened. Although they knew Sasha was going to her first rave, Keith said, drug use was "not her character."
When they got the call that Sasha had been taken to the hospital, they had no idea how serious her condition was.
"We were almost like joking, 'Oh she's going to be in trouble. I can't believe she went there!' " she said. Keith said they thought maybe the rave was "too packed, she was excited and passed out.... Never in a million years would we would have imagined" drugs.
A 16-year old friend who was with Sasha at the rave said Sasha was dancing, got hot and began quickly drinking cold water.
Doctors said "her sodium, electrolytes were so low that when she started replacing them so quickly [with cold water], ecstasy messes up your body's ability to process that, so it threw her body out of whack," said Keith.
Sasha passed out, hitting her head on the floor. The friend who was with her tried to shield her body but told her family that the venue was so crowded, people stepped on them.
"We've got kids in inappropriate situations that are dying," Keith said. "That's a city-, state-, county-owned building. They throw it, they rent it, yet they fill it full of minors in a setting where everybody knows what a rave is about. Where's the accountability?"
On Monday, Dr. Caitlin Reed, a physician from the U.S. Centers for Disease Control and Prevention on assignment with the Los Angeles County Department of Public Health, said she was at the rave on both days and did not see anyone checking identification.
"I didn't see any ID being checked at the entry point," Reed said. "This was an all-ages event. There were many, many younger teenagers present."
Pat Lynch, who manages the Coliseum, was stunned to learn of the death Tuesday.