In the song, "Ode to Billie Joe", what did Billie Joe McAllister throw off the Tallahatchie Bridge?

This is an excellent story song. I remember when Bobbie Gentry was interviewed on the Mike Douglas Show and was asked what was thrown off the bridge. She said that it was unknown.

I read that it was their still born baby. Billie Joy couldn't handle how they desposed of it, so he killed himself.

Beautiful song.