I think that something we don't know is how much time elapsed between Tom's move to Las Vegas (assuming he did) and the massacre. He was present at the mall for Tessio's and Carlo's denouement. If one of Michael's and Vito's objectives was to keep Tom out of things, they sure blew that.

Also, I agree with DT that Tom was just plain too immersed in Corleone family criminal matters and for too long for the FBI to have been mislead about any legitmate role. Still, as TB points out, once Tom completed his legitimate role in Nevada, Michael brought him back to assist with the day of reckoning.