Officials have identified all the hijackers, and estimated there were three to five for each of the four passenger planes involved. At least two of the hijackers were on the Immigration and Naturalization Service "watch list," and it's still unclear whether the individuals entered the United States illegally or whether they entered before their names were placed on the list.
Most if not all of the hijackers were Egyptian or Saudi nationals, sources said (link via Steven)

I think we should still be cautious about how watertight this evidence is, but the reports are piling up. I hope none of these conclusions are premature, because I bet the strike is in motion right now. I have no illusions that there won't be one.