Moxley's body was found after a night of partying with Skakel, his older brother Tommy and other teenagers in an affluent gated community in Greenwich, Connecticut.
Authorities said she was bludgeoned and stabbed to death by a broken golf club found near her body.
For more than two decades, the case languished.
A series of books on the high-profile crime renewed interest, leading to new tips and a new suspect in January 2000: Michael Skakel, who was 15 at the time of the killing.
He turned himself into police after an arrest warrant was issued, all the while proclaiming his innocence.
Kennedy told CNN on Wednesday that five witnesses saw Skakel 11 miles away at the time of Moxley's death.
"He has an airtight alibi," Kennedy said, "but unfortunately, he was very poorly represented."