Another answer

I asked another forum if Fredos men could have killed the shooters and one of the guys gave a detailed explanation.

"That is exactly who killed the assassins zaf. I was gonna post about this a few days back, but completely forgot because I was very busy.

In the screenplay for The Godfather: Part II it says the assassins (three of them; in the movie only two) had their throats cut when they were found in the ditch. That is why there were no further shots fired after the initial attempt on Michael. Before reading the screenplay I thought maybe Fredo used a silencer and shot them dead, but if their throats were cut it means it was not Fredo. Two things now. First, whoever did it had to be very handy with the knife, and Fredo certainly wasn't. The second thing is whoever did it probably got blood on themselves, and on their clothes, and needed to get out of the scene quick and go clean up. He wouldn't have time to change and therefore couldn't be outside pretending to be looking for the assassins.

That also means it wasn't Rocco or Neri or any of their men because they were at the scene immediately and they also would need time to go clean up. A missing capo would be way too suspicious. That rules out Rocco or Neri (who was dealing with Klingman at that moment.) Now, their men are well chosen and well paid and would have no reason to turn on the family. On top of that their own capos would notice their absence. So that rules them out too. Therefore, logic suggests that since it wasn't the capos or Fredo it could only have been one (or more) of Fredo's men. When Fredo was with his hysterical wife he would have no time to oversee his own men. It would be easy for any or some of them to dispose of the knife and clothes while Fredo was busy dealing with his wife.

When Fredo was approached by Johnny Ola it was very probable some of Fredo's men were approached as well. It would make sense on the part of Johnny Ola to do that. He was very shrewd and knew Fredo did not command the respect of the Corleone soldiers. It would be easy to buy off some of his men who were greedy enough or felt they stood no chance at being promoted or were not close enough to Michael to care about turning on him. Whatever the reasons, this is someone whose identity the movie never tells us and we're likely never to find out." - Hammerstrike



THIS ISNT MY WORK BUT SOMEBODY ELSES WHO POSTED IT ANOTHER FORUM.

LINK: http://www.imdb.com/title/tt0071562/board/thread/81264474?

This is the most likely answer to who killed the shooters IMO.