Cedar Hill police have issued an Amber Alert for a 12-year-old girl they believe is "in grave or immediate danger."
Sommer Brown is a black girl who is 5 feet 6 inches tall and weighs 90 pounds. She has black hair and brown eyes and was last seen wearing a black tank top and black basketball shorts.
Police believe she is with her mother, 43-year-old Lisa Elizabeth Brown, a black woman who is 5 feet 8 inches tall and weighs 120 pounds. She has black hair and brown eyes and was last seen wearing a black blazer and a light green shirt.
Police said they believe the pair may be traveling in a gray/pewter 2006 Ford Expedition with a paper Carmax tag that reads 86P 3357 and expires on May 19, 2013.
"I'm worried for her safety and her well-being and ... we need to find her ASAP ... because I don't know what her mother could do to her," said Madison Dews, Sommer Brown's stepsister.
Cedar Hill police said Lisa Brown, an Atlanta schoolteacher who is currently on spring break, came to North Texas to try to get temporary custody of her daughter.
The girl's father was awarded custody about three weeks ago.
Police said Lisa Brown filed a police report accusing her estranged husband of sexually abusing his daughter. Cedar Hill police said they looked into the claims, found they were false and arrested Brown on suspicion of filing a false police report on Tuesday.
Brown bonded out of a jail on Wednesday and sought an emergency temporary custody order through family courts on Thursday but was denied, police said.
After the court denied her custody, Brown tried to take her daughter out of school, but the school denied her request because she doesn't have custodial rights, police said.
Investigators said Lisa Brown drove to her estranged husband's neigbhorhood in the 100 block of Hannah Circle at approximately 6 p.m. on Thursday and called her daughter.
Dews said she was sitting with her stepsister inside the house while she talked with her mother on the phone.
Her mother told her stepsister "how much she missed her, she couldn't live without her, and she wasn't going to live without her, and that she wasn't going to stop trying to get her," Dews said.
She said Sommer Brown went outside "because her mother begged her for a hug."
When Sommer Brown finally came out of the house, neighbors told police they heard Lisa Brown yelling at her daughter to get in the car. When she finally complied, Brown sped away, witnesses said.
Dews said she ran after the car, with her stepsister staring at her through the window.
"She was, like, looking at me like to see what our next step was going to be ... because she knew that she wasn't planning on leaving with her mother," Dews said.
Family members say their top concern is getting Sommer Brown home safe.
Dews said she wants to know that her stepsister safe and wants her back home. She also said she wants her stepsister to know that her family is looking for her, saying that she knows Sommer Brown doesn't want to be with her mother.
"She had no intentions on ever seeing her mother again," she said.
Police said Lisa Brown is not cooperating with police and has cut off contact with her lawyer, who was trying to get her to turn herself in and return her daughter.
Lisa Brown has ties to Houston, Atlanta and Chicago, according to Cedar Hill police.
Anyone with information on the Browns' whereabouts is asked to call the Cedar Hill Police Department at 972-230-5785.
NBC 5's Amanda Guerra contributed to this report.