Here's a tale for you. I have two dogs I adopted from a rescue agency -
Keena and Stefi. Both are females who were both about 4-5 years old when I
met them. Keena is never a problem. I took her to dog training classes and
applied the same techniques to Stefi when I got her.

Stefi is about 45 pounds and when you look closely at her ears you'll see
lots of scar tissue from old dog fights. After I adopted her, I found out
about her past. She had been kept in a kennel at a no-kill animal shelter
for almost two years because she was "too aggressive". No one would adopt
her. Today, a lot of people who know her laugh because she is so
ridiculously affectionate with strangers (including kids). However other
dogs are another story.

In my town there are a lot of leash free dog parks and my dogs love to
visit. We go almost everyday.

I've been working hard trying to better socialize Stefi and we've made lots
of progress. I've noticed that when Stefi finds herself in a crowd of people
and dogs all bunched together she gets very nervous. Her smiling face will
suddenly disappear, her eyes turn coal black, she snarls and the
aggressively lashes out at a nearby dog (especially smaller ones). If I see
it, I intercept and stop it by shouting "be nice" to her. It seems to work.
But I can't take my eye off of her for even a second. I've also noticed
after she lashes out at a dog, she never does it again to that dog.

Today, after a year and a half of trying to socialize her, she bit another
dog in the face so badly, the owner had to take his dog to an emergency vet
clinic. It was a bloody mess.

Fortunately, the dog wasn't seriously injured but I got to pay a $60 vet
bill for the owner. The victim dog was a small breed (Italian greyhound?)
Stefi has previously caught and killed a rat and baby opossum in my backyard
and some small breeds are not much larger.

I'm now very hesitant about taking her back to the park. And what about my
other dog who was not involved. Do I leave Stefi at home and take Keena? Any
ideas for training regimens? Remember this is a leash free area that's quite
large.

Any help with my problem is greatly appreciated.