Karl Kesel has published a story on his FB page about a 10 page Challengers of the Unknown story that he and Art Adams did back in 2000. Kesel has the whole account but the short version is that neither he nor Adams ever got the art back from DC. However it was found for sale on a dealer’s website. While the dealer took the art down for sale when contacted by Kesel, he said he had gotten it from another dealer and didn’t return it.
So the art remains in a gray area. Kesel would like it returned but it’s provenance remains unknown. DC is very scrupulous about art returns so the whole story is a bit of a mystery. But there it is.