But the suspect they arrested late last week didn't fit their expectations.
Police say Sandra's killer is 28-year-old Melissa Huckaby, a Sunday school teacher and the mother of one of Sandra's close friends. Huckaby allegedly killed the second-grader and dumped the suitcase containing her body in a nearby irrigation pond.
Though little is known about Huckaby or a possible motive, if the allegations are true, Huckaby would not fit the typical profile of a killer. According to the Justice Department, roughly one in 10 homicides are committed by women. And when women kill, their victims are more likely to be someone close to them, like their children, boyfriends or spouses.
Forensic psychologists and criminal profilers say women who kill have backgrounds and motivations that are often quite different from their male counterparts. Compared with men, women are more likely to be related to their victim, less likely to plan in advance and less likely to use extreme violence.
Criminologists: Women and Men Kill for Different Reasons
"Women are different in whom, how and why they kill," said James Alan Fox, a criminologist at Northeastern University. "The victims are younger, they're more often related to them, they kill with means other than guns.
"The traditional female role is a nurturer, not a murderer. Extreme violence is far more alien to females than to males," he added. "When a murder is committed by a female, it's more likely to be self-defense or can reflect some sort of mental illness."
Though women committed roughly 10 percent of murders between 1976 and 2005, they were involved in nearly 35 percent of murders of intimate partners and nearly 30 percent of murders of where the victim was another family member, according to the Justice Department.
So far, police have not released a possible motive for Cantu's death. Prosecutors told the Associated Press Monday that they are considering rape and molestation allegations against Huckaby. San Joaquin County Deputy District Attorney Robert Himmelblau told The AP Monday that a homicide charge against Huckaby could also include the special circumstances of rape with a foreign object, lewd and lascivious conduct with a child and murder in the course of a kidnapping.
Motive in Sandra Cantu Slaying Still Unclear
Sgt. Tony Sheneman declined comment on whether or not Huckaby may have shared any details about how Sandra died. He said autopsy results would yield further information when they are released, likely in several weeks.
Family members said Huckaby, the granddaughter of the local pastor, was a devoted mother to her 5-year-old daughter.
"We're very, very shocked," said Joani Hughes, Huckaby's aunt. "It's very out of character for Melissa."
But her father, Brian Lawless, said that his daughter had often struggled with raising a child on her own and had sometimes suffered from bouts of depression.
"I have no doubt in mind she knows right from wrong," her grandfather, Clove Road Baptist Church pastor Lane Lawless, said. "At least I thought she did. Maybe I'm mistaken."
Brian Lawless said he's visited his daughter in jail and that they cried together.
"She looks tired," he said. "She hasn't slept much, but all in all, she looks good."
But when he sees his daughter's mug shot on television and hears the crimes she's accused of he doesn't see the woman he raised.
"The young lady I see on film – that's not my daughter," Lawless said.
Huckaby had also been in trouble with the law. When she was arrested Friday night on kidnapping and murder charges, after six hours of questioning, she was on probation for theft.
Huckaby does not have any history of violent behavior, Sheneman said, adding police are confident that she acted alone.
Huckaby remains in police custody and is due to be arraigned in court today.
Police said that when Cantu was captured on a surveillance camera, happily skipping down the street, she was on her way to play at Huckaby's home. Sheneman said police believe the 8-year-old was killed "very soon after she was seen on the video."
Dr. Michael Welner, a forensic psychiatrist and chairman of The Forensic Panel, told "Good Morning America" today that the arrest of a woman in a murder such as Sandra's is "really strange and peculiar."
"There's more to the story," he said. "And instinct tells me to sit back and let the case come to all of us."
Welner said there are a "whole range of possibilities" that could have led to Huckaby's arrest.
"The possibilities are she is as horrible as everybody imagines," Welner said. "Or she could have gotten in over her head in an interrogation and gave statements that incriminated herself in a way that are actually inaccurate."
Welner said women are statistically less likely to murder than men in part because "destruction and the ability ... use power to knock someone down is so tied into a masculine identity."
"That's not part of what makes women feminine in this culture or in any culture," he said.
Though there is no single profile of female killers, mental illness has played a part in many of their high-profile killings.
According to Jack Levin, a criminology professor at Northeastern University, female killers are more likely than men to suffer from serious mental illness, while male killers more typically suffer from personality disorders, and are often sociopaths who are unable to empathize with their victims.
Andrea Yates, a Houston mother who drowned her five children in 2001, was found not guilty by reason of insanity and sent to a state mental hospital. Her attorneys argued that she suffered from severe postpartum psychosis, believed Satan was inside of her, and was trying to save her children from hell.
Women Less Likely to Use Extreme Violence
"There are a number of different roads that lead to someone taking someone else's life," said Xavier Amador, a clinical psychology professor at Columbia University. "In women, in my experience, it tends to be typically related to a history of abuse or psychosis."
Women are also less likely to use a violent means of killing or to plan out their attacks, psychologists say. Though women account for roughly 10 percent of all homicides, they are involved in 36 percent of poisonings, according to the Justice Department.
Stacey Castor was sentenced to 25 years to life last year for poisoning her husband David Castor with antifreeze, and to another 25 years for the attempt to kill her daughter Ashley Wallace, then 20, with an overdose of drugs and vodka in September 2007.
"Women tend as a rule to do softer killings, poisoning, suffocation, those sorts of things. Rarely are they the slasher types or inflicting a lot of bodily damage," said Gregg McCrary, a retired criminal profiler for the FBI.
"For women, killing is often seen as a last resort -- a defensive move, whereas, for men, it's an offenseive move," said Fox.