6:49 AM PT -- TMZ Sports spoke with a person from Trent's camp who tells us the NFL star is ADAMANT the neighbors dog attacked one of his dogs first -- and his dogs only acted in self defense.
Williams says the neighbors dog was attacking his daughters' tiny Basenji dog -- a smaller, more fragile dog. At that point, a larger pit bull mastiff came over to assist in the fight, and got a hold of Sandy. Killing her in the process.
The point Williams is trying to make ... is that his pitbulls weren't running rampant in the neighborhood, attacking random pets, but rather, this was the result of an unfortunate animal altercation.