A body can be burned to ashes only. In the Joseph Duncan case he built a regular fire in the national park when he murdered Dylan Greone after kidnapping, raping, and torturing him for 6 weeks.
When he was caught and Shasta took them back to the location all they could find were ashes of his little body and teeny tiny bone fragments.
I have seen other cases where a suspect was able to burn their victim/s up leaving ashes only. If they put her body in a burn barrel and used igniters like old tires and lighter fluid it would cause tremendous heat and the body would be reduced to small bone fragments and ashes which by now are probably gone. If they had a spaded shovel they could continue to break up the bones if they built a bonfire over her body making them tinier and tinier until they were nothing but small teeny fragments or ash. (It pains me to even write that)
Unfortunately Tara nor Dylan are the first victims to be burned to ash by the monster who murdered them. Its so common for those who live in the South to have and see bonfires going in rural areas late at night so even if someone saw a bonfire blazing late at night they wouldn't find it suspicious. IMO