(CNN) -- Investigators expanded their search into Texas Monday for a 7-year-old girl who was reported missing after her mother was found dead in Oklahoma Sunday night.
Authorities also are trying to find the dead woman's estranged husband, the Oklahoma State Bureau of Investigation said in a statement.
The girl -- Aja Daniell Johnson -- was reported missing from Geronimo, Oklahoma, late Sunday, the bureau said.
Her mother, Tonya Hobbs, 37, was found dead inside a parked RV Sunday night, the bureau said. Hobbs and her daughter were visiting Hobbs' estranged husband, Lester William Hobbs, at his sister's home, the bureau said.
Lester Hobbs, 46, and Aja are missing and are presumed to be in Hobbs' car, said investigators who issued an Amber Alert to try to find her. Lester Hobbs is not Aja's biological father, police said.
Texas authorities also issued an Amber Alert, Texas Department of Public Safety spokeswoman Lisa Block said. Authorities believe Hobbs has connections to Rockport, Texas, and the alert was issued preemptively, she said.
Tonya Hobbs and her daughter were last seen Saturday night. Authorities did not say how Hobbs died, but the bureau said investigators believe someone killed her.
The RV belonged to Lester Hobbs, and was parked at his relatives' home, Richard Goss, agent in charge of the state bureau of investigation's Lawton, Oklahoma, office, told reporters Monday. Hobbs' relatives became suspicious Sunday after they did not see Tanya Hobbs or Aja, and one of them pried open the door and discovered Tanya Hobbs' body, he said. Lester Hobbs and Aja were gone, Goss said.
The Hobbses were separated, but apparently Tanya Hobbs was visiting Lester Hobbs in an attempt to reconcile, Goss said. The couple had been together for about five years.
A medical examiner was performing an autopsy, the bureau said, and police want to question Lester Hobbs about the death.
He and the girl are believed to be traveling in his car, a white 1992 Toyota Paseo with Oklahoma tag number 577-BPW. The two-door car has no hubcaps, and the rear passenger window is covered in plastic, the bureau said.
The Comanche County, Oklahoma, Sheriff's Office said Aja has a medical condition that requires medication.
"Our main concern at this time is to find the little girl," Sheriff Kenny Stradley told reporters. "We know that she does need some medication," and authorities were checking leads regarding that, he said. He did not specify Aja's medical condition.
Goss said Lester Hobbs has an extensive criminal history but did not elaborate. The Sheriff's Office said he had prior felony convictions.
Lester Hobbs was supposed to appear in court in Lincoln County, Oklahoma, on a DUI charge, Goss said, and did not appear, so a warrant has been issued for him.
Aja's biological father was awarded emergency custody of her in November, according to Oklahoma County District Court documents obtained by CNN affiliate KWTV of Oklahoma City, Oklahoma. At a hearing, Tonya Hobbs -- identified as Tonya Dunkin in the documents -- and the girl's father, John Johnson, agreed that she would have supervised visitation with Aja and keep the girl away from Lester Hobbs, the documents said.
Aja is 4 feet tall and weighs 59 pounds, Goss said. She has brown eyes, and her hair is brown with the ends dyed black.
Lester Hobbs is about 6-foot-1 and 190 pounds, with hazel eyes and brown hair, according to the Oklahoma State Bureau of Investigation. The agency released a picture of him and said he has a mustache but no longer has a goatee.
Geronimo is about 12 miles south of Lawton, Oklahoma, in the south-central part of the state.