(CNN) -- Relatives of slain California 8-year-old Sandra Cantu expressed "shock" Saturday at news that a neighbor -- a Sunday school teacher whose daughter was a playmate of Sandra's -- was arrested in the girl's death.

Melissa Huckaby was charged with kidnapping and murder in the death of 8-year-old Sandra Cantu.

"Just utter shock and disbelief," Sandra's aunt Angie Chavez told CNN affiliate KOVR. "I can't imagine a mother doing this to a child."

Melissa Huckaby, 28, was booked into the San Joaquin County Jail early Saturday and faces charges of kidnapping and murder in the death of Sandra in Tracy, California.

Sandra was last seen March 27 in the Tracy mobile home park where she lived with her family -- the same mobile home park where Huckaby lives.

The girl's body was found Monday, stuffed into a suitcase submerged in a pond at a dairy farm.

"We have to live the rest of our lives without Sandra. [Huckaby] is in jail. She can still see her little girl grow up," Sandra's uncle, Joe Chavez, told KOVR.

He said the family is "shell-shocked" and hopes that in time they can learn to trust people again.

"Who can you trust at this point?" he said. "Who do you know?"

A police spokesman told reporters Saturday he "couldn't begin to theorize" a motive for Sandra's death. Watch police discuss the arrest »

Huckaby is the granddaughter of Clifford Lane Lawless, pastor at the Clover Road Baptist Church, which was searched as part of the investigation, Tracy police Sgt. Tony Sheneman told reporters. Huckaby taught Sunday school at the church, he said.

In addition, Huckaby's 5-year-old daughter was close friends with Sandra, who often played at Huckaby's home.

Earlier Saturday, Sheneman said he did not believe Huckaby had retained an attorney.

In a Friday interview with the Tracy Press newspaper, Huckaby acknowledged owning the black rolling suitcase that held Sandra's body, but said she reported it missing the day before the girl's body was found. She said it disappeared the same day the girl was last seen.

"There's been a lot of speculation on the news about what happened to my suitcase," she told the newspaper. "It's not my granddad's. It's mine, and someone took it."

Police said Huckaby lives in the same mobile home complex where Sandra lived with her family; the Tracy Press reported Huckaby lives with her grandparents.

Police noted "inconsistencies" between comments Huckaby made to the newspaper and a previous statement she gave authorities, Sheneman said. It was one reason police asked her Friday to come to the station to be interviewed again. She drove there about 6 p.m. Friday, he said.

Authorities believe Huckaby lied about the suitcase being missing, Sheneman said.

Asked whether authorities believe Sandra's death was planned, Sheneman said the question is "not something I can answer right now" but "we do have an indication." He said authorities have found where they believe the girl was killed, but would not divulge that location to reporters.

"I couldn't begin to theorize what her motive is," Sheneman said of Huckaby.

During the Friday night interview with police, Huckaby was "very relaxed for a bit, and then she became very emotional, and then she became relaxed again" before appearing resigned, Sheneman said. He declined to say whether Huckaby had confessed, but said "she revealed enough information that we had probable cause to arrest her."

Huckaby was booked into the San Joaquin County Jail about 3:25 a.m. (6:25 a.m. ET), according to the county sheriff's Web site. She is set to appear in court Tuesday.

Police took Huckaby into custody about 11:15 p.m. Friday (2:15 a.m. ET Saturday), Sheneman said, but she chose to continue talking with them for more than two hours.

Authorities told Sandra's family of the arrest about 2:15 a.m. local time, he said. "They were in disbelief." Watch neighbors remember slain girl »

Police have had contact with Huckaby before, but she does not have a record of violence, Sheneman said, but did not elaborate. He said no other arrests are expected in Sandra's death.

Citing court records, the Tracy Press reported Huckaby pleaded no contest January 9 to a felony charge of second-degree commercial burglary and a misdemeanor charge of property theft with a prior theft, burglary or robbery.

A criminal complaint says she was jailed in Los Angeles County on a property theft conviction in 2006 and that she attempted to steal from a store in November 2008. According to the complaint, she is on probation in San Joaquin County and is due back in court April 17.

Asked about the court records, Huckaby told the newspaper it wasn't her and that she did not know why the address and cell phone number in the court documents match her own. "I don't know what to say," she said. "That's not me."

Police believe that by the time Sandra was reported missing, she was already dead, Tracy Police Chief Janet Thiessen told reporters Saturday.

"There was some speculation early on that [the suspect] would be a man," Sheneman said Saturday. "It's unusual for a woman -- statistically, according to the FBI -- to be involved in anything like this."

The day Sandra was last reported seen, she returned home from school, kissed her mother and left to play with a friend who lives nearby.

A short time later, wearing a pink Hello Kitty T-shirt and black leggings, she left to go to another friend's home, according to a family spokeswoman. Police said the girl's clothing helped them identify her body.

Huckaby told the Tracy Press that Sandra stopped by her home that day to ask if she could play with Huckaby's daughter, but Huckaby refused because she wanted her daughter to pick up her toys. Sandra left for another friend's home, Huckaby told the newspaper.

How Sandra died has not been made public. An autopsy was conducted earlier in the week, but CNN affiliate KRON-TV said results are not expected for weeks. Thiessen said Saturday any autopsy information as well as search warrants filed in the case remain under seal.

All About Police • Murder and Homicide • San Joaquin County