When 8-year-old Sandra Cantu of Tracy, Calif., disappeared in March 2009, the small community was set on edge. The family's worst fears were confirmed when Sandra's body was found in a suitcase in an irrigation pond not far from where she lived. She had been raped and murdered, and everyone wondered what kind of monster could do that to an innocent young girl.

As it turned out, the monster was a local Sunday school teacher, a 29-year-old mother named Melissa Huckaby whose daughter was a friend of Sandra's. She pled guilty to first-degree murder and faces 25 years to life in prison without the possibility of parole.

The case is bizarre and sensational, but how rare is it?

The answer depends on what specific part of Huckaby's crime you're talking about. The fact that the child was murdered by someone she knew and trusted is not unusual at all. Because of alarmist news coverage, the general public often believes that the greatest threat to children is from strangers.

In fact, the vast majority of crimes against children are committed not by convicted sex offenders or strangers, but instead by the victim's own family, church clergy, and family friends.

According to the National Center for Missing and Exploited Children, "Based on what we know about those who harm children, the danger to children is greater from someone they or their family knows than from a stranger."

A 2000 report by the Office of Juvenile Justice and Delinquency Programs reported that over three-quarters of kidnappings were committed by family members or acquaintances of the child. The study also found that children abducted by strangers were harmed less frequently than those taken by acquaintances.

Children are in far more danger of being abused, kidnapped, or killed by their parents than any stranger on the street.

Nor is Huckaby's gender particularly unusual. Depending on which study you look at, mothers kill children at least as often as fathers do.

According to a report titled "Homicide Trends in the U.S." issued by the Department of Justice's Bureau of Justice Statistics, of all children under age five murdered between 1976 and 2005, 31 percent were killed by fathers and 29 percent were killed by mothers.

A 2000 study conducted by Marlene Dalley, Ph.D., analyzed trends in Canadian child murders and concluded that "Both mothers and fathers kill infant children at the same frequency, though when all (victim) age groups are considered mothers and step-mothers killed more children than fathers and step-fathers."

A 2003 study reported in the Journal of the American Medical Association found that of 34 North Carolina newborns who were killed or left to die, at least 85 percent of them were killed by their mothers (usually through strangulation or drowning).

In fact, the risk of a person being murdered - probably by his or her mother - is 10 times higher during the first day of life than at any other time. This of course flies in the face of the public's perception of women as protective nurturers, but the fact is that women murder children far more often than people realize.

The strangest aspect of the Sandra Cantu case is, of course, the sexual assault. Details of the crime, including the sexual nature of Huckaby's assault, have been suppressed under a gag order from Superior Court Judge Linda Lofthus, so the circumstances aren't available to analyze.

Women killing or physically abusing children (their own or other people's) is not unusual, but a mother who rapes a young girl is quite rare. Most women who molest children do not abduct or kill them, and most forcible rapes are committed by men.

This tragedy may serve to remind us that monsters come in many forms - even a mother at a local church.