Sources say Melissa Huckaby had the girl in a nearby park. The seven-year-old was reported missing and, the sources add, returned, then found to have been drugged.
Huckaby is accused of abducting, sexually assaulting and slaying Sandra Cantu, a playmate of Huckaby's young daughter.
The families of Huckaby, Cantu and the seven-year-old all live in the same trailer park in Tracy, Calif.
Huckaby is now on suicide watch behind bars.
As sources spell out the timeline to reporter David Begnaud of CBS station KOVR in Sacramento, Calif., a female neighbor of the Cantus called police to say her daughter was missing on January 17 at 5:17p.m. local time.
At 5:31 p.m., police reported that the girl was with Huckaby at a nearby park.
At 5:39 p.m., the girl was returned, and police left without filing charges.
Later that night, the girl was taken to a local hospital, where doctors told police she had tested positive for a muscle relaxing drug -- raising questions, Begnaud says, of whether she'd been drugged by Huckaby.
But, Begnaud adds, sources say police couldn't prove that was the case because five hours had passed from the time the child was dropped off to the time she was tested -- enough time for the girl to have gotten the pills from someone other than Huckaby.
On the night she was arrested, Huckaby told CBS News she had in fact taken the little girl to a nearby park, but claimed she had permission to do it.
The evidence apparently coming to light "may very well" help prosecutors' case against Huckaby, CBS News legal analyst Lisa Bloom told co-anchor Julie Chen Tuesday. " ... In California, prior similar incidents can come into a trial -- we saw it in the Phil Spector case, we saw it in the Michael Jackson case -- if it's sufficiently similar to show a motive or a pattern. We have a little girl, seven years old, close in age to Sandra Cantu, who is eight years old. Same location, in the trailer park. And maybe the same M.O., abducting her and drugging her. We don't know if (drugging is) alleged yet in the Sandra Cantu case, but may very well be.
" ... What the prosecutors are going to want to see if they can prove to the judge (is), is it sufficiently similar? Do the facts line up in a way to show that she's got a pattern of this kind of criminal behavior?"
The Huckaby defense team "should definitely be worried" about the latest developments, Bloom says, "and they're going to be looking to show that it's dissimilar. This girl maybe just wandered off on her own. ... They're going to want to show that the (drug) in that little girl's system perhaps came from somewhere else. It's a common ingredient in a lot of cold medications for children. Maybe she got it from a parent or from somebody else in the trailer park. They're going to want to show it's completely different than this situation."
When Chen mentioned speculation that Cantu's death may have been accidental, Bloom replied, "Keep in mind that Melissa Huckaby is also charged with sexual assault. So it doesn't sound like an accidental situation. And she's charged with murder. Murder, of course, is an intentional act, premeditated, probably not accidental."