Washington ? The nation’s worst blackout may have resulted from a combination of power plant shutdowns, line failures and voltage problems that were building for several hours before the final crash, according to a sequence outlined Friday by a U.S.-Canadian task force.

The panel’s initial findings provide no precise cause for the Aug. 14 blackout that affected 50 million people or say why it was allowed to cascade over such a wide area in two nations. They make clear investigators will have to broaden their effort to include events possibly hours before the failures in FirstEnergy’s grid in Ohio that have been a focus of public attention.

“What we see so far is that a voltage collapse caused plants and power lines to disconnect. But that doesn’t answer why those things happened,” Energy Secretary Spencer Abraham cautioned as the task force released a detailed timeline.

The task force said most events that appeared to have contributed to the blackout occurred between noon and 4:13 p.m. EDT, when the breakdown was in full force.

But the report also said many events happened well before that “across several states” and remained of interest to investigators.

These events should not be discounted, said Jimmy Glotfelty, a member of the task force.

The finding appears to reinforce a contention made repeatedly by Akron, Ohio-based FirstEnergy Corp., that the failure of three of its high-voltage transmission lines south of Cleveland in the hour before the blackout could not in themselves have triggered the massive power outage.

“It acknowledges the importance of looking at all of the events in order to determine how they may be related, ultimately to determine how they may have caused the outages,” said FirstEnergy spokeswoman Ellen Raines.