June 11, 2011  -- It's a theory that could eliminate one of the prosecution's key pieces of evidence in the murder trial of Casey Anthony: Someone tampered with the body of Caylee Anthony, the 2-year-old girl allegedly murdered by her mother.

The murder trial taking place in an Orlando, Fla., courtroom is largely built on circumstantial evidence. One of the most important pieces of evidence are strips of duct tape that were found on Caylee's skull. Their location indicates that the tape was placed over the toddler's mouth and nose, suggesting the child was suffocated.

Caylee's remains, found six months after she was last seen alive, were badly decomposed. The medical examiner concluded her death was a homicide, but could not determine a cause of death.

In his opening statement, lead defense lawyer Jose Baez argued that Roy Kronk, the meter reader who discovered Caylee's body, tampered with the remains.

"Mr. Kronk is a morally bankrupt individual who actually took Caylee's body and hid her," Baez charged. Kronk later "found" the body, Baez claims, because he hoped to collect a reward.

Kronk, who has denied Baez's accusation, is expected to be called as a witness later in the trial.

DePaul University law professor Andrea Lyon, the former co-lead defense counsel for Casey Anthony, said that if someone did move Caylee's body before alerting authorities, much of the evidence -- including the duct tape -- should be considered "legally irrelevant."

"The physical state of the remains are questionable because it appears that the remains were moved," she said in an exclusive interview with "20/20." "And that means that whatever's on them does not indicate that that's the way that the child was at the time of her death."

Evidence photos in the Casey Anthony murder trial

Lyon said the prosecution "cannot prove that the remains were not moved or that duct tape wasn't placed there later."

In his opening statement, Baez said that Kronk told his son in November 2008 that he had found Caylee's body. Kronk didn't report the body to authorities, Baez said, until a day after he received a $1,000 bill related to car trouble in December.

"He called his son and he said, 'I'm going to be famous, I'm going to be rich. You know, watch the news, I'm going to be on the news,'" Lyon said.

That call, she said, was made "before he found the body in December, and he took a day off and then just decided to wander into this area that had been searched and searched and searched... and he finds the body," Lyon told "20/20." "That's very suspicious."

Caylee's remains were found in an area that had been searched several times earlier, Baez said in his opening statement.

The toddler's body, he said, "was placed there to be found, not to be hidden."

Baez did not explain in his opening statement how Kronk allegedly came into possession of Caylee's body.

Kronk said in 2008 that he "discovered and reported to my management and appropriate authorities the remains of a human body located in a wooded area."

Recently, a lawyer for Kronk released a statement rejecting Baez's claims, saying: "This defense theory regarding Mr. Kronk is completely false. The suggestion that Mr. Kronk took possession of Caylee's remains is totally lacking in logic or explanation. It did not happen."

The prosecution has not directly addressed the allegation that the body was tampered with but forensic anthropologist John Schultz testified Friday that Caylee's vertebrae were found all in the same place, a sign that the body was intact when it was brought there. He estimated that the body may have been there as long as six months.