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Eight-year-old Robin Lemus of Glendale was found with her foster parents' 1998 Lexus ES 300 sedan around 10 a.m. She told police she was left in the car and told to stay put, but the others never returned.
Police say they do not know where the 12-year-old girl, biologicial mother, Iris Zavala, or the unidentified male are.
Robin and Estephanie of Los Angeles were last seen around 3 a.m. Monday. They went missing, along with the family's Lexus sedan.
Authorities say the 12-year-old knows how to drive; they thought the girls took the car out for a joy ride.
Police are not calling this case an abduction, but they believe the biological mother of the 12-year-old is with the girls.
Iris Zavala, 38, is believed to have helped orchestrate the young girls' runaway, police said. Zavala is the only biological parent they have no been able to contact.
"She is a person of interest right now," said Lt. Roxanne Hart of the L.A. County Sheriff's Department. "This, at this point, didn't rise to the level of an Amber Alert, but it does rise to the level of putting out a request for anybody who sees this vehicle or the girls to call right away," Hart said.
The birth father of Robin Lemus, Felicio Lemus, told Eyewitness News that police searched his Glendale home Monday morning. He also said his daughter text messaged him several times last week, complaining about the treatment at the foster home.
"My daughter called and she was crying, because she said the foster mom didn't give her dinner. I called her back around 9:30 and she was crying. I just told her to be patient and to see if she could get some water or something. And she said she was not allowed to get out of her room," Lemus said.
A neighbor said he saw the girls get into the Lexus hours earlier than 3 a.m.
"I saw the 8-year-old, but I wasn't sure the 12-year-old wasn't an adult. I thought she was an adult. So I didn't really think anything of it. I saw them get into the car around midnight," said Nick Patton.
Patton said the girls got into the car through the back passenger side door, as if they were trying not to be seen.
"They were sitting inside the car for about 15 or 20 minutes and started the car up. And I went back inside because I didn't think anything of it," Patton said.
Anyone with information is asked to call the Los Angeles County Sheriff's Department at (909) 450-2711.
Eyewitness News Reporters Gene Gleeson, Robert Holguin and Lisa Hernandez contributed to this report.