A time line released by a U.S.-Canadian task force did not provide any new indication on what precisely triggered the Aug. 14 blackout which investigators believe started with power line problems in Ohio.
The blackout began to cascade from Ohio and Michigan into southern Canada and New York state at about 4:11 p.m., investigators have said previously.
"Most of the events that appear to have contributed to the blackout occurred during the period from about noon to about 4:13 p.m.," the task force said Friday.
But "many things happened well before 12 p.m." on Aug. 14 and some of those problems also "may be relevant in a causal sense to the blackout," it added.
The task force said unusual transmission problems were observed "well before 12 p.m. ... across several states."
"An apparent voltage collapse" on the day of the blackout occurred on parts of the transmission systems around and within the northern Ohio and Eastern Michigan grid, the task force said.
However, it said more work needs to be done before a cause of the massive outage — and particularly what caused it to spread so widely and so quickly — can be determined.
"While we are making good progress, this investigation is far from complete," Energy Secretary Spencer Abraham said in a statement.
The blackout, the worst in the nation's history, cascaded from Ohio and Michigan across southern Canada and New York state and knocked out electricity to 50 million people. At one point more than 100 power generating stations were shut down because of the transmission voltage surges.
The task force said it was looking into whether the voltage collapse may be tied to a lack of "reactive power" — a portion of the energy moving through power lines that is essential for keeping proper voltage levels and balance.
"Sufficient voltage is maintained by supplying the transmission system with reactive power from generating stations and static devices called capacitors," said the task force. "...When reactive power is limited, the increased loading will cause a voltage drop along the line."
Canadian Natural Resources Minister Herb Dhaliwal, who with Abraham co-chairs the task force, said that the sequence of events released Friday is "only a first step in the process of uncovering what happened."
The initial task force findings reflected timelines that generally had been made public previously by investigators and by FirstEnergy Corp., the company that has been at the center of the investigation because of a power line failure in its northern Ohio system during the hour before the blackout.
By F. Josef Hebert