I agree with Turnbull.

I think it is entirely reasonable to suppose that Rocco would have spoken with (ie. interrogated) all of the men in the compound to find out who heard what, who saw what, and when. As head of security he would be derelict in his duty not to do so, or even more so. If people from Fredo's crew carried out the killings, it would have been found out. Why wasn't it found out? Because Rocco wouldn't have spoken to Fredo. Not the bosses brother. He would have been above suspicion. It all points to Fredo being the killer, which everyone agrees is ridiculous.

As I wrote, it's a massive failing in the plot construction.