The Department of Energy released the results of their initial investigation in to the blackout. The report provides a timeline of events affecting the major transmission lines of the power grid from about noon until the actual blackout occurred.
What I found most interesting is two events that were natural causes that, if implicated as playing a large role in either the cause of the blackout, or at least in widening the scope, would highlight the fragility of the system. At 2:02 PM, a 345 kV transmission line in southwestern Ohio disconnted because of a brush fire. At 3:23 PM, another 345 kV transmission line between northern and eastern Ohio (one of three that disconnected during this time frame) disconnected due to contact with a tree. Did these two lines have any role? I don’t know, but I would like to see the conclusions of the report to find out.