Associated Press

Callers have inundated the phone lines of Tracy police, saying it can't be. Veteran homicide and sex-crime researchers say they cannot recall a case quite like it. Even the investigators themselves looked at the evidence and initially said "no way."

A woman was accused not only of killing someone else's child, but of raping her

Law enforcement officials and other experts say the allegations against Melissa Huckaby in the slaying of 8-year-old Sandra Cantu are remarkably rare over decades of U.S. police work.

Huckaby was charged Tuesday with murdering her daughter's playmate, with the added special circumstances of rape with a foreign object, lewd or lascivious conduct with a child under 14 and murder in the course of a kidnapping. The 28-year-old divorced mother is due back in court Friday, when she is expected to enter a plea.

Sandra's body was found on April 6 — 10 days after she went missing. It was stuffed in a suitcase that was pulled from an irrigation pond near Tracy, a small town where San Francisco's suburbs meet California's farm belt.

Tracy police Sgt. Tony Sheneman said dozens of callers a day have insisted that Huckaby could not have acted alone, that no mother would rape another's child, that the scenario was too improbable to be true. The case is so striking that police initially shared the public's reaction.

The investigators themselves, when first confronted with the evidence that pointed to Huckaby, were inclined to look for another suspect.

"When investigators were first looking at this they went 'Huh, no way... Who did she work with?'" Sheneman said. "We got that info and said 'there's no way, that doesn't happen.'"

"After this case, I'll never say never again," Sheneman said, adding that police remain confident that Huckaby acted alone.

Department of Justice data on U.S. homicides dating back more than 30 years highlight the unusual nature of this crime, said James Fox, a criminologist at Northeastern University.

Of the more than 600,000 cases recorded — more than 90 percent of U.S. homicides since 1976 — only one comes close to the alleged circumstances of Sandra's killing, said Fox. The data did not include names and some other details; the Associated Press was unable to locate the case.

Researchers say the Huckaby case does not match the typical profile of sex crimes by females.

Women represent only 1 percent of all adult arrests for forcible rape and 6 percent of all adult arrests for other sex offenses, according to a Department of Justice report.

When they do commit sex crimes, women often are acting as accomplices to men, and their victims tend to be teenagers, said David Finkelhor, director of the University of New Hampshire Crimes against Children Research Center.

"It's very, very rare for women to molest children, and when they molest children it's very unusual for them to molest a child of this age," Finkelhor said. "It's unusual for women to kill children who are not their own."

Police have declined to publicly state where and how Sandra was killed, but they have said they do not have a motive.

"I find it really hard to speculate on the motivation," Finkelhor said.

Court documents and interviews with family members show Huckaby had a rocky personal life. She went through a divorce and bankruptcy and fought depression as she tried to hold down a job and raise a child.

In 2002, she won a restraining order against a boyfriend who had an extensive criminal record and a restraining order from a previous marriage, according to San Joaquin County court records.

She married John Huckaby in 2003, separated a year later and divorced in 2005. In divorce papers, Melissa Huckaby accused John Huckaby of child abduction, domestic violence and alcohol abuse — allegations he denied in an interview with ABC's "Good Morning America" on Friday.

Records show she was arrested in November and charged with burglary and petty theft from a store. The judge suspended the case and appointed a doctor to assess Huckaby's mental health. She was found competent to stand trial. In a deal with prosecutors, she pleaded no contest in January to the petty theft charge and the burglary charge was dropped.

Huckaby's attorneys could use the sheer statistical improbability of the murder case to cast doubt on the allegations, regardless of the evidence, legal experts said.

"Instinctively it doesn't feel like a good fit," said William Portanova, a Sacramento defense lawyer and a former state and federal prosecutor.

"It's an extraordinarily rare circumstance to have an adult female commit a sexual assault and murder on a female child alone," Portanova said. "So right off the bat, any attorney is going to be looking to disprove that theory."