The explosive allegations are contained in some 500 pages of new evidence from witnesses and detectives in a report on Caylee's killing released by the Florida State Attorney's Office. The report includes damning letters to the inmates, and word on conversations she supposedly had with those inmates, reports CBS News Correspondent Betty Nguyen.
Caylee was last seen in June 2008. Her disappearance wasn't reported until a month later. Casey was charged with first-degree murder that October. Caylee's body was found in December 2008 in a wooded area less than a mile from the Orlando, Fla. home she and Casey shared with Casey's parents. Casey has pleaded not guilty, claiming a babysitter kidnapped Casey. Prosecutors say they'll seek the death penalty if Casey is convicted of first-degree murder.
The documents quote the two inmates as saying Casey told them she sometimes drugged Caylee when she wanted to go out and party.
Robyn Adams, who's serving 10 years on drug charges, told investigators she and Casey exchanged letters, and Casey wrote that she used "stuff" to put Caylee to sleep.
Adams' interview with police went further. In a transcript, Adams says Casey used antihistamines on Caylee, and a detective's summary of that same interview says Casey used chloroform.
That could be significant, notes Nguyen, because police say they found traces of chloroform -- a powerful sedative -- in the trunk of Casey's car and in a syringe found near the child's body.
Adams says Casey also told her that Caylee's remains were found wrapped in a blanket inside a garbage bag. But that information wasn't public knowledge at the time Casey supposedly said it.
In the report, an Orange County detective says, "Casey Anthony had knowledge of items only the suspect … law enforcement … and the medical examiner knew."
Casey's defense team countered by saying. "The letters .. clearly demonstrate Casey's unconditional love for her daughter, Caylee."
Defense lawyers also questioned the motive of the two convicted criminals who say Casey befriended them, saying, "It is obvious … (the) only goal in corresponding with Casey Anthony was to create leverage to get out of prison early."
Casey's long-awaited trial is slated to begin in next year.
How damaging could those letters be? Are they admissible?
CBS News legal analyst Lisa Bloom addressed those and other aspects of the case on "The Early Show" Wednesday: