Who killed the assassins?

That would be the only logical explanation, but the reason it irks me is because in earlier posts back in the Imdb days there was a theory floating around that Rocco was part of the betrayal but Michael didn't want to reveal that he knew this until he knew he had Hyman Roth on the ropes and then he has Rocco go on a suicide mission to rectify his betrayal.

best explanation I've heard - sort of like the bank robbery at the beginning of Dark Knight.
3 guys on the hit squad, one of them kills the other 2 and either gets out (using inside-info of the compound from Fredo) or does a good job of hiding (or maybe Fredo hides him?!) until it's safe to leave - once Michael's men find the 2 bodies, they'll stop looking for anyone else.
Having rewatched last night, you don't hear any further gunshots after the attempt, and the dead men look like they've had their throats cut. So definitely not Fredo.
One interesting thing I hadn't noticed before though was Fredo's wife (Deanna?) is the one who "discovers" the bodies and starts screaming "Mike, they're dead. They're right outside my room". What if this was a diversion while Fredo helped hide or smuggle out the 3rd assassin?

That said, when Michael's talking to Tom and says that they wont be found alive and that there's been a betrayal, Tom starts saying "you don't think Rocco or Neri...?" and Michael says that they're only connected to him by business so why not.
So he clearly suspected anyone but Tom (and ironically Fredo and Connie by extension!).

No, and Fredo is too incompetent to carry out a double-cross hit on the assassins. He wasn't even good at keeping a low profile in Cuba when he was transporting Michael's money in that suitcase as well as keeping his betrayal a secret when they went out later to party with the Senator.

How do the assassin's go about slitting their own throats?