STOCKTON, Calif. -- A Sunday school teacher who pleaded guilty to kidnapping and murdering a neighbor girl in Northern California made a tearful apology to the victim's family Monday as she was sentenced to prison for the rest of her life.

The punishment came after Melissa Huckaby, 29, reached a plea deal with prosecutors that took the death penalty and the possibility of parole off the table in the killing of 8-year-old Sandra Cantu of Tracy, whose body was found in a suitcase pulled from an irrigation pond.

Huckaby offered no motive for the 2009 killing as she addressed the girl's mother, Maria Chavez. The victim was a playmate of Huckaby's daughter.

"I still cannot understand why I did what I did. This is a question I will struggle with for the rest of my life," Huckaby said, her voice trembling.

She said Sandra "did not suffer, and I did not sexually molest her."

"I'm asking you, Maria, for your forgiveness," Huckaby said. "I can't imagine forgiving someone who harmed my daughter. I hope someday you can forgive me."

Later in the day, prosecutors said Sandra had died of asphyxiation.

In a summary of their investigation, prosecutors said authorities found a rolling pin with a bent handle and a bloody smudge matching Sandra's blood inside the church where Huckaby had taught.

Authorities had released few details about the killing until Monday, when Judge Linda Lofthus lifted a gag order. Prosecutors released a summary of their investigation that showed the cause of Sandra's death was "homicidal asphyxiation."

The summary included a pathologist's report saying she was found with a piece of bloody cloth tied around her head. Some of her injuries were consistent with a rolling pin with a bent handle and bloody smudge found inside a church where Huckaby had taught. The smudge matched Sandra's blood.

The victim's body also contained alprazolam, a prescription sedative commonly used to treat anxiety. Prescription bottles of alprazolam were found in Huckaby's purse and home, prosecutors said.

Huckaby entered her surprise plea last month to first-degree murder with the special circumstance of kidnapping. All other charges, including allegations that she sexually abused the girl, were dropped as part of the plea deal.

Sandra's family asked the court to play a video depicting her short life. The footage brought many in the courtroom to tears, including Huckaby.

"She changed the lives of a lot of people. A lot of people," said Daniel Cantu, Sandra's father, who broke down as he addressed Huckaby.

"All I can say right now is, repent and think about what you've done," he said.

Sandra's aunt Angie Chavez told Judge Linda Lofthus that Huckaby robbed her family of the ability to see Sandra grow up, go to college, fall in love, get married and have children.

"No amount of explanation can make any sense of what she did or why she did it." Angie Chavez said. "The only solace we have as a family is to know that she will never do this again."

Authorities have released few details about the March 2009 killing.

At the sentencing, Lofthus lifted a gag order that had been in place since last year. She was set to consider whether to unseal documents later in the day.

Sandra was last seen on a surveillance video walking through the Orchard Estates Mobile Home Park on March 29, 2009. Prosecutors said Monday the girl can be seen looking in the direction of Huckaby's home before she goes off-camera.

Eight minutes later, the tape shows Huckaby driving her SUV from the mobile home park in the direction of the church where the bloody rolling pin was found.

A massive search ended April 6, 2009, when her body was found stuffed in a black suitcase pulled from the pond a few miles from the community where she lived.

A couple later told authorities that they saw Huckaby and her SUV at the pond on the day of Sandra's disappearance, and the woman told the couple that she was there to urinate.

Huckaby was arrested less than a week later after telling a reporter the suitcase belonged to her but had been stolen from her driveway the day Sandra disappeared.

Huckaby showed authorities a handwritten note that she claimed she found at the trailer park that reads, "Cantu locked in stolin suitcase thrown in water onn Bacchetti Rd. & Whitehall Rd witness."

The handwriting on the note was similar to Huckaby's, prosecutors said.