Olivant: We know their throats were cut. If you watch the scene on the big screen, you can see the slice marks across the necks.

The big problem with this scene, they left the bodies in front of Fredo's place. But we all know "he has a good heart, but he's weak." No way Fredo had the cajones to do the deed. Plus their guns were placed across the chests. What? they didn't fight back or shoot the oncoming assailant? They knew him (her)!