Police arrested 28-year-old Melissa Huckaby, a neighbor whose own daughter was one of Sandra's playmates.
Huckaby is a Sunday school teacher, and granddaughter of the pastor of a nearby church that was searched extensively Friday by police investigators.
Soon after that search, Huckaby drove herself to the police station, where she was questioned until early Saturday morning.
Sources tell CBS News that Huckaby told four different stories of what happened the day the 8-year-old girl disappeared - inconsistencies that prompted further inquiries from investigators.
"She gave enough information to us during the course of the interview that probable cause was there to arrest her," Police Sgt. Tony Sheneman told the Associated Press.
CBS Station KPIX reports that at 2:25 a.m. Saturday, police drove Huckaby out of their headquarters in an unmarked car, headed for the San Joaquin Jail, where she was booked on charges of kidnapping and murder.
She is being held without bail, with arraignment set for Tuesday afternoon.
"We have information that by the time (Sandra Cantu) was reported missing to us, she had already been murdered," Police Chief Janet Thiessen told KPIX.
"I couldn't begin to even theorize what her motive might have been," he said.
Sheneman said that during the course of her interview with police, Huckaby's emotions ranged from appearing relaxed to very emotional, and ultimately "resigned" to the course of events.
In a phone interview on Friday, Huckaby told KPIX that Sandra visited her home March 27, the day she disappeared. Huckaby also said she had left a suitcase in the driveway that day, and that it was missing.
Sandra's body was found 10 days after she disappeared, stuffed into a suitcase and dumped in a farm pond.
Huckaby's family had been questioned at length during the investigation, and their home and vehicles had been searched, Sheneman said.
Sheneman said investigators had not expected that the suspect would turn out to be a woman.
"It's unusual for it to be a woman statistically and according to the FBI," he said at the news conference.
Discovering it was a woman and a member of the tight knit Tracy community who knew the family was "a double blow," he said.
"Today's going to be a very difficult day for everyone to digest that," Sheneman said. "This was an anomaly in the murder of a child."
"There's still a lot of work to be done in the next several weeks to ensure that Miss Huckaby pays for what's she's done," Sheneman said.
Cantu's disappearance had , wondering how a little girl could just vanish so close to home.
The last time her mother saw Sandra alive, the little girl was going outside to play with friends, and surveillance video taken in their trailer park showed Sandra happily skipping on the trailer park's grounds.
The slain girl's aunt, Angie Chavez, said in a phone interview with The Associated Press early Saturday that she was happy to learn of the arrest.
"I want to know why she did it, if she did it," Chavez said. She added that she had no indication that Huckaby could be a suspect.
At an early morning news conference, Tracy Police Chief Janet Thiessen said investigators had worked on the case tirelessly.
"We have information that Sandra, by the time she was reported missing to us, that she probably had already been murdered," said Thiessen.
"It has helped us to bring Sandra home, again not in the way that we would've hoped, but that was out of our hands shortly after she went missing."
Lawless did not immediately respond to a call seeking comment Saturday.
The Tracy Press reported that Huckaby was released Thursday from Sutter Tracy Community Hospital, where she spent several days in intensive care for what she described as "internal bleeding."
KPIX also reports that Huckaby was recently convicted for petty theft in San Joaquin County. According to court records, the judge ordered her to be examined by physicians after Huckaby claimed she was not mentally competent to stand trial. It was determined that she was competent and she pleaded guilty.
Huckaby was also convicted in November of 2006 for petty theft at the Bellflower Superior Court in Los Angeles County.
Huckaby was scheduled to appear in court on April 17 to check in with a county mental health program as part of a three-year probation sentence for a petty theft she pleaded no contest to.
In an interview with The Tracy Press on Friday, Huckaby said someone else by the same name was facing charges for the attempted November theft from Target.
To listen to KPIX's phone interview with Huckaby click on the video player below.