PROVO, Utah -- Authorities say a U.S. woman accused of killing six babies that she gave birth to over 10 years told investigators that she either strangled or suffocated the children and then put them inside boxes in the garage of her home in the western state of Utah.

According to a probable cause statement released by police Monday, Megan Huntsman said that between 1996 and 2006, she gave birth to at least seven babies and that all but one of them were born alive.

Huntsman, 39, said she killed them immediately after they were born, and put their bodies inside the boxes. The statement said each baby was wrapped in either a towel or a shirt, and placed in a plastic bag.

Huntsman is being held on $6 million bail -- $1 million for each baby she's accused of killing. It wasn't immediately clear if she had an attorney.

Huntsman was arrested Sunday on six counts of murder after police found the infants' tiny bodies in her former home in Pleasant Grove, a leafy, sleepy town about 35 miles (56 kilometres) south of Salt Lake City. A seventh baby found appears to have been stillborn, Utah County Attorney Jeffrey Buhman said.

During the interview with police, Huntsman was unemotional and matter of fact, according to Pleasant Grove police Lt. Britt Smith.

Formal charges have not yet been filed against Huntsman and no other arrests have been made but Buhman said the investigation remains open.

Investigators were trying to determine if the seven babies had the same father or multiple fathers, Buhman said.

The gruesome case has raised a series of questions about how the killings occurred despite Huntsman carrying out what neighbours said seemed like a normal existence.

"How can you have a baby and not have evidence and other people know?" asked neighbour SanDee Wall. "You can't plan when a baby is going to come. Just the thought of somebody putting a baby into a box is a heartbreaker."

Police declined to comment on a motive and on what Huntsman said during an interview with investigators.

Her estranged husband, Darren West, who had recently been in prison on drug-related charges, made the discovery Saturday with fellow family members while cleaning out the garage of the house, which is owned by his parents. He called Huntsman, who admitted to him it was her baby, according to court documents.

West called police, who then found the six other bodies in the garage, police Capt. Michael Roberts said.

Late Sunday, West's family issued a statement saying they were in a "state of shock and confusion."

The babies' bodies were sent to the Utah medical examiner's office for tests, including one to determine the cause of death. DNA samples taken from the suspect and her husband will determine definitively whether the two are the parents, as investigators believe.

Investigators believe West and Huntsman were together when the babies were born, but don't believe he was aware of the killings. Buhman said Huntsman is the principle suspect, but didn't rule out more arrests as the investigation continues.

Smith declined to say if Huntsman gave birth to the babies in the home or at a hospital.

Huntsman also has three daughters -- one teenager and two young adults -- who lived at the house.

Police have talked with West as they investigate his level of knowledge and involvement in the deaths, Smith said. He was living in the house during the decade that authorities believe Huntsman had killed the babies, Smith said.

He's been co-operative, and was devastated by the discovery, he said.

Smith said the three daughters have been interviewed, but he declined to discuss what they said.

West pleaded guilty in federal court in 2005 to two counts of possessing chemicals intended to be used in manufacturing methamphetamine, according to court records. In August 2006, he was sentenced to 9 years in prison, but appealed three times. He maintained his innocence and said he never had any intention to manufacture meth.

Huntsman wrote a letter asking a federal judge to consider leniency at sentencing.

"Darren is a remarkable man, husband, brother, son, son-in-law, friend and father of our three beautiful daughters," she wrote, continuing, "Please we need this guy to keep our family together."

West was released from a federal prison in California in January and transferred to a halfway house in Salt Lake City, said Chris Burke, spokesman for the Federal Bureau of Prisons.

Neighbour Sharon Chipman said the couple married young, and Huntsman never worked except for a short stint at a grocery store.

The three daughters who were living in the house were good young women who have turned out remarkably well considering their father has been in prison, Chipman and Wall said.

Cheryl Meyer, a psychology professor at Ohio's Wright State University, said some women who kill their children hide or deny their pregnancy and then dispose of the baby after it's born. Meyer said "concealers" are typically teenagers who do not repeat the act.

"These are usually girls who are 17, get pregnant, become scared to death and don't want to tell their parents," said Meyer, who has written about mothers who kill their children. "They're not 30-year-old women who can go have an abortion."

To combat this, all states, including Utah, have safe haven laws that allow women to drop off unwanted newborns to authorities with no questions asked. The mother can remain anonymous as long as the child has not been subject to abuse or neglect.

In coming days, defence attorneys for Huntsman are likely to closely examine her background to search for any evidence of mental illness or a family history that would help explain the alleged killings, said George Parnham, who represented Andrea Yates, the Texas woman who drowned her five children in her bathtub in 2001.

Defence attorneys also will try to determine whether Huntsman sought an abortion and if she told anyone about her pregnancies -- all in hopes of understanding actions that otherwise appear incomprehensible, Parnham said.

"You start off with the very nature of what happened. Is there a rational motive?" he said.

- Associated Press writers Michelle Price in Salt Lake City and Matthew Brown in Billings, Montana, and news researcher Rhonda Shafner in New York contributed to this report.