Call a trainer without delay!
Labs are at the top of the list for dog bites.
My brother had a Lab that he thought was a very friendly dog and the dog almost killed my dad. Totally out of the blue. My Dad was afraid of dogs , however that cannot be an excuse. You must take this seriously and see a trainer before someone gets hurt. In WI , if you are aware of aggression and the dog kills someone you will be charged with a minnimum charge of manslaughter. I have owned many Labs and have never seen any aggression on their part. Sometimes this is a traight that is passed on and other times it is a learned behavior. I hope you can get a handle on this problem for your sake and the dogs.