SPRING, Texas - A Texas woman accused of killing a young mother and abducting her 3-day-old boy had suffered a miscarriage and intended to "adopt" the newborn as her own, authorities said Wednesday.

Verna McClain, 30, is charged with capital murder after confessing to killing the mother and grabbing Keegan Golden outside a pediatrician's office in suburban Houston. Keegan was found unharmed hours later with McClain's sister.

The Montgomery County Sheriff's Office said Wednesday that McClain - estranged from her husband, with whom she had raised three children - had told her fiance that she had given birth to his child. Instead, McClain had miscarried, Capt. Bruce Zenor said.

McClain's sister, Corina Jackson, told authorities that she had talked about needing to "do the adoption" soon after taking Keegan.

Her fiance, who was not identified, is being interviewed by authorities.

Officials said they did not believe anyone else was involved in the shooting and abduction.

Sheriff Tommy Gage said Kala Golden had placed Keegan into her pickup truck Tuesday afternoon after leaving Northwoods Pediatric Center in Spring, about 20 miles north of Houston. The shooter was parked next to her, Gage said.

During the confrontation, the woman repeatedly shot Golden, then snatched the child from her truck and drove away, according to witness accounts. The dying woman leaned into the vehicle and tried to take the boy back, screaming, "My baby!" but her attacker sped off.

Two detectives spotted a vehicle later Tuesday outside a nearby apartment complex that matched witnesses' descriptions, Gage said. Though McClain's apartment was empty, she later arrived and talked to authorities.

"During her interview with detectives, information was obtained which led detectives to a residence in Harris County where her sister lives and a possible location of the child," Gage said. McClain was later arrested. Police said that she admitted carrying out the attack.

"I don't understand that, her taking a child," said her estranged husband, Theo McClain of San Diego. The couple had two children together and raised a third from a previous relationship of his.

"That doesn't make sense. We have three kids. And then to shoot somebody? I've never known her to hurt anybody."

Investigators said that McClain's statements included information only the shooter would know and indicated she attacked the mother Tuesday as part of a wider plan to kidnap any child and that Golden was simply a convenient target.

The baby has been returned to family members, according to Keith Schuchardt, who said he had been married to Kala Golden for three years.