Michael Vick -- Grand Champion Pit Bull Euthanized After Dogfighting Career

Sad news in the dog world -- the top pit bull from Michael Vick's dogfighting ring has been euthanized.

The dog -- named Lucas -- was humanely euthanized Wednesday at the Best Friends Animal Society in Kanab, Utah because of accumulating health problems. He was around 13 years old.

Lucas -- who was Vick's grand champion -- was one of nearly 2 dozen dogs that were rehabilitated at BFAS after they were rescued from the ring in 2007. Unfortunately, Lucas was not allowed to be adopted because of his vicious fighting history -- and therefore had to remain a lifetime-care dog at the animal sanctuary.

We're told Lucas was one of the friendliest dogs at the sanctuary -- and never stopped grinning.