First, the reason Michael didn't suspect Tom--why he told him, "Right now you're the only person I can completely trust," was that a) Tom's loyalty was never in question; and b) since Michael had deliberately kept Tom out of the Roth negotiations, and since Michael had concluded that Roth was behind the shooting, he had no reason whatsoever to link Tom with Roth and the attempt. Rocco and Neri, on the other hand, sat in on the meeting with Johnny Ola and may have had motivations to help get rid of Michael. And, as Michael told Tom, "Fredo...is weak and stupid, and this is life and death," which I took to mean that even then, Michael may have felt Fredo had something to do with the attempt, even if it was inadvertent.
But a question that occurs to me is: Since Rocco was linked to Michael, shouldn't Michael have been concerned that if Rocco were killed or captured at the airport, the trail would lead right back to Michael? Michael was a careful man, and I doubt he would have left a loose end like that. I'm guessing that, even though Rocco had been a visible member of his entourage for years (including very visible service as Michael's security guy during Anthony's party), Michael probably had made provisions to portray Rocco for the record as some kind of independent security contractor so that he could have plausible deniability.