Police said it's possible she met an unknown male on MySpace and left to meet up with him. Her older sister Maria Gutierrez was at the party that night when she said Karina asked her for unusual advice.
"She just asked me this simple question: Would you leave with somebody that you don't know?" Maria said. "I said, 'No, you don't know him. I wouldn't leave with nobody that I don't know."
The teen had a MySpace account, and her family said they feared she may have run with another teen or an adult male that she met online.
Her mother said she doesn't know the young man her daughter may have willingly left the neighborhood with. It's one possibility in her disappearance, though police haven't ruled out kidnapping.
The 7th grader was known to frequent Sylmar Park and the Sylmar Public Library. She is described as Hispanic, 5 feet, 3 inches tall, about 90 pounds, and was last seen wearing a white Madonna T-shirt and black shorts.
Anyone with information is urged to call the LAPD Mission Hills Division at (818) 838-9800.
As they wait for good news, the family said they love her and just want her back safe.