Authorities have charged a woman with killing mom-to-be Bobbi Jo Stinnett (search) and cutting the fetus from her womb.

Lisa M. Montgomery (search), 36, of Melvern, Kan., was charged with kidnapping resulting in death. She is accused of showing up at Stinnett's house under the pretext of buying a dog, then strangling her and taking the woman's premature baby girl.

According to a criminal complaint, Montgomery admitted to the crime. The complaint also said Montgomery lied to her husband about giving birth, although U.S. Attorney Todd Graves declined to give a motive for the crime.

Montgomery was in possession of the infant believed to be the stolen fetus when police found the baby on Friday, ending a day of frantic searching. DNA tests will confirm whether the girl is Stinnett's.

Montgomery, a mother of two, had been pregnant but lost a child, Graves said, though it was unclear when or under what circumstances.

Missouri State Highway Patrol Sgt. Sheldon Lyon said earlier in the day that authorities were questioning a man and a woman who were in the place where the baby was found. Graves said the investigation was ongoing but would not say if additional charges might be filed or if there was another suspect.

Graves said Montgomery contacted Stinnett through an online message board, and authorities zeroed in on her using computer forensics. Montgomery was seeking to buy a dog from Stinnett, who raised rat terriers, he said.

According to the criminal complaint, Montgomery's husband, Kevin, told officers he received a call Thursday from his wife, who said she was in Topeka, Kan., about 40 miles from Melvern, and had gone into labor and given birth.

Kevin Montgomery and the couple's two children met Lisa and the newborn at a parking lot in Topeka and drove home, according to the affidavit.

The kidnapped baby, said to be in good health, was taken to a hospital in Topeka, Kan., authorities said.

An adult man and woman were at the home where the baby was found. A red Honda hatchback matching the description of the one authorities were hunting for was parked in front of the house.

"We’re going to cancel the Amber Alert — we’re that confident that we have the girl taken from Skidmore [Mo.]," said Nodaway County (search) Sheriff Ben Espey at a Friday evening press conference. "We may have not ever recovered this little baby if the Amber Alert system was not put into place. We’re happy, we’re very happy."

Authorities said the baby was being held overnight for observation but was doing well. They weren't commenting on details of the investigation, other than to say that it was ongoing.

Earlier Friday, investigators put out the Amber Alert (search) for the missing fetus and had expanded their search outside Missouri, where 8-months-pregnant Stinnett, 23, was found slain in her house by her own mother Thursday afternoon.

Espey said authorities are awaiting DNA testing to confirm the girl is Stinnett's child.

"It's about as good as we can get, people," Espey said.

Espey believes Stinnett, a factory worker married a year and expecting her first child, was likely strangled and resisted the attack. Her husband was at work at the time and is not a suspect, he said.

Authorities hoped that strands of hair found in the dead mother's hands will help find the killers.

"The autopsy is going to show us there was some blond hair probably found in her hands," the sheriff said. "That would also help us with the DNA."

Authorities knew they needed to rescue the premature baby quickly, hoping to find the infant in time to offer much-needed medical attention.

"Ninety-five percent [of fetuses] are going to do pretty well given a modest amount of medical attention," said Dr. Perry Clark, medical director of the neonatal intensive care unit at The University of Kansas Hospital.

Espey said investigators were looking for a car that had been reported seen in the driveway at the Stinnett home Thursday afternoon, a red two-door Honda hatchback from the late 1980s to early 1990s.

He said that at about 12:45 a.m. officers in Atchison County saw a car that appeared to match that description and began chasing it, but lost sight of it after its lights were turned off.

Espey said he had been in contact with FBI agents from Virginia who are specialists in abduction of babies. Although he declined to say specifically where investigators were looking, only that their focus will be on states adjacent to Missouri.

The Missouri Major Case Squad (search), the Missouri State Highway Patrol (search) and an investigation team from neighboring Buchanan County were assisting Nodaway County officials in the investigation.

An autopsy was to be conducted in Kansas City by the Jackson County medical examiner.

Espey said investigators knew Stinnett was still alive within an hour of being found around 3:30 p.m. Thursday. Paramedics tried to revive the young woman, who was pronounced dead later at a hospital.

Espey said there were no visible signs of struggle on the victim's body, and no indication of forced entry into her small white home in this small community of about 500, located north of St. Joseph in the extreme northwest corner of Missouri.

The sheriff said Stinnett worked at Kawasaki Motors Manufacturing (search) in Maryville, was married little more than a year and had been expecting her first child.

Chris Smock and his wife Elaina went to school with Bobbi Jo Stinnett and live right behind the Stinnett's house. The couple, who found out about the killing while they were at work, said they were "shocked and horrified."

"It's not your average everyday thing, of course," Chris Smock said. "I think Skidmore will react the same way any town would react. I mean, how would you react?"

Several pregnant women have been killed in recent years by attackers who then removed their fetuses, in some cases to pass the children off as their own.

In the most recent case, a 21-year-old woman was shot to death in Oklahoma in December 2003, allegedly by another woman who pretended the 6-month-old fetus was her child. The fetus died and prosecutors are seeking the death penalty.

FOX News' Carol McKinley, Jeff Goldblatt, Catherine Donaldson-Evans and The Associated Press contributed to this report.