Investigators spent five days searching dense vines and marshes for a missing infant, only to find her lying quietly in a small cedar box hidden under her baby sitter's bed.

Clothing was packed around it to muffle any sounds and baking powder placed inside to mask the stench of dirty nappies.

Authorities say the baby's mother, Chrystina Lynn Mercer, gave her to baby sitter Susan Elizabeth Baker on Saturday, then reported her missing about 10 hours later.

Sheriff Bobby Haddock choked up on Thursday as he described how 7-month-old Shannon Dedrick was stashed in the box for 12 straight hours before investigators discovered her on Wednesday. They believe she had been in the box on and off for several days before that.

"She was way back under the bed," he said. "But she was not crying."

Baker had written a letter to Governor Charlie Crist's office in August, pleading for help for the baby and claiming her father shook her and both parents did drugs in front of her.

She asked Mercer on Friday if she could have permanent custody, Haddock said. Officials do not believe Shannon's father, who is Baker's stepbrother, was involved in the disappearance.

Haddock said Shannon apparently had been fed and cared for while she was with Baker, who lived about 19 kilometres from Mercer, though there was no bottle in the box with her. He said Baker became a suspect several days ago but never told them where the baby was, even as they interviewed her for 12 hours while Shannon remained hidden.

Mercer and Baker were jailed and arraignments were scheduled for Thursday. Mercer was charged with interference of child custody, desertion of a child and several other charges. Charges against Baker included neglect of a child with aggravated circumstances and interference of child custody.

Baker's husband, James Arthur Baker, was arrested but released. He is still under investigation, Haddock said.

Shannon's parents told investigators they last saw her when they went to bed around 3am on Saturday and investigators thought she had vanished sometime between then and 8am.

About 100 law enforcement agents and others spent days scouring dense vines and marshes around the baby's home in a remote, makeshift community of dirt roads, tin-roof shacks and old mobile homes. Searchers also dug through rubbish cans and dumpsters.

According to court documents, child welfare officials began looking into allegations Shannon was being abused less than two weeks after she was born.

Investigators frequently went to the infant's home from August to late September and reported that both parents used marijuana and kept a messy home. But they said Shannon seemed to be cared for and in September, a physician determined she was healthy.

Susan Baker was involved in another missing child case in South Carolina more than two decades ago. She told authorities her stepson, 3-year-old Paul Leonard Baker, disappeared from the family's home on March 5, 1987, while she napped.

A massive manhunt turned up nothing. She and her husband were extradited to South Carolina in 2000 and charged with assault and battery in Paul's disappearance, according to police reports. But a grand jury never indicted them and the child was never found.

Susan Baker did serve prison time after authorities investigating her stepson's disappearance discovered a 6-year-old girl in the Baker home had been badly beaten. She was sentenced to 10 years in prison but the sentence was suspended after 80 days. Authorities could not say how she was related to the girl.