Casey Anthony's father believes his granddaughter, Caylee Anthony, was drugged, he said in a Dr. Phil interview that aired Wednesday.

George Anthony doesn't know whether Casey drugged her daughter, as prosecutors alleged, or whether someone else did it -- but he doesn't believe Casey's story that Caylee's death was an accidental drowning.

I believe that Casey or someone else that she was with possibly gave too much to Caylee, she fell asleep and didn't wake up, he told interviewer Phil McGraw. Asked what he thought Caylee was drugged with, he suggested a prescription drug like Xanax, which is used to treat anxiety, or possibly chloroform, which is what prosecutors argued Casey used.

Dr. Phil asked George Anthony why he thought Casey would have killed her daughter, and he gave the same explanation as the prosecutors: To go out and have a good time, to be with friends, to have this life that she didn't have with Caylee.

Caylee Anthony's body was found on Dec. 11, 2008, five months after she had been reported missing by Casey's mother, who said Casey's car smelled like a dead body and told police she had not seen her granddaughter in a month. Casey, now 25, told investigators a variety of stories, including that Caylee had been kidnapped by a nanny who turned out not to exist. Prosecutors alleged that Casey had killed Caylee with chloroform, covered her mouth with duct tape and buried the body; Casey claimed her daughter had drowned accidentally and that she had panicked and hidden the body. But Caylee's body was too badly decomposed for the medical examiner to prove or disprove either story.

In July, Casey Anthony was acquitted of murder in a verdict that shocked many of the people who had followed the intensely public trial. She was convicted of four misdemeanor charges for lying to investigators and sentenced to four years in jail, but the judge gave her credit for time served and she was released shortly thereafter.

Casey's mother, Cindy Anthony, seemed very surprised by her husband's answer, telling Dr. Phil that she had never heard him say he believed Casey had drugged Caylee. She added that she does not suspect her daughter.

Reuters reported that Cindy Anthony saw Casey's acquittal as a sign from God that she was innocent.

I believe that justice for Caylee was when her mother walked, she told Dr. Phil. I believe that in all my heart.

The Anthonys' interview with Dr. Phil was split into two parts, the first of which aired on Tuesday.