6:49 AM PT -- TMZ Sports spoke with a person from Trent's camp who tells us the NFL star is ADAMANT the neighbor's dog attacked one of his dogs first -- and his dogs only acted in self-defense.
Williams says the neighbor's dog was attacking his daughter's tiny Basenji dog -- a smaller, more fragile dog. At that point, a larger pit bull mastiff came over to assist the Basenji and in the process got a hold of Sandy -- killing her.
We're told as a result of the initial attack, Trent's daughter's dog is hospitalized with serious injuries.