The Ohio utility under scrutiny in the blackout, FirstEnergy Corp., also said Sunday there were problems, including strange voltage fluctuations, in the Midwest power grid hours before its transmission lines failed.
What began as a handful of commonplace, summertime "trips" - brief transmission line shutdowns, usually due to ebbing voltage caused by anything from a bird hitting the lines to a power overload - appears to have set off the biggest outage in U.S. history. Fifty million people lost power in eight states and parts of Canada.
Though some problems began earlier, the actual blackout did not begin until shortly after 4 p.m. EDT Thursday. Some key moments:
2 p.m. FirstEnergy's Eastlake Unit 5, a 680-megawatt coal generation plant in Eastlake, Ohio, trips off. A giant puff of ash from the plant rains down on neighbors. On a hot summer afternoon, "that wasn't a unique event in and of itself," says Ralph DiNicola, spokesman for Akron, Ohio-based FirstEnergy. "We had some transmission lines out of service and the Eastlake system tripped out of service, but we didn't have any outages related to those events."
3:06 p.m. FirstEnergy's Chamberlin-Harding power transmission line, a 345-kilovolt power line in northeastern Ohio, trips. The company hasn't reported a cause, but the outage put extra strain on FirstEnergy's Hanna-Juniper line, the next to go dark.
3:32 p.m. Extra power coursing through FirstEnergy's Hanna-Juniper 345-kilovolt line heats the wires, causing them to sag into a tree and trip.
3:41 p.m. An overload on First Energy's Star-South Canton 345-kilovolt line trips a breaker at the Star switching station, where FirstEnergy's grid interconnects with a neighboring grid owned by the American Electric Power Co. AEP's Star station is also in northeastern Ohio.
3:46 p.m. AEP's 345-kilovolt Tidd-Canton Control transmission line also trips where it interconnects with FirstEnergy's grid, at AEP's connection station in Canton, Ohio.
4:06 p.m. FirstEnergy's Sammis-Star 345-kilovolt line, also in northeast Ohio, trips, then reconnects.
4:08 p.m. Utilities in Canada and the eastern United States see wild power swings. "It was a hopscotch event, not a big cascading domino effect," says Sean O'Leary, chief executive of Genscape, a company that monitors electric transmissions.
4:09 p.m. The already lowered voltage coursing to customers of Cleveland Public Power, inside the city of Cleveland, plummets to zero. "It was like taking a light switch and turning it off," says Jim Majer, commissioner of Cleveland Public Power. "It was like a heart attack. It went straight down from 300 megawatts to zero."
4:10 p.m. The Campbell No. 3 coal-fired power plant near Grand Haven, Mich., trips off.
4:10 p.m. A 345-kilovolt line known as Hampton-Thetford, in Michigan, trips.
4:10 p.m. A 345-kilovolt line known as Oneida-Majestic, also in Michigan, trips.
4:11 p.m. Orion Avon Lake Unit 9, a coal-fired power plant in Avon Lake, Ohio, trips.
4:11 p.m. A transmission line running along the Lake Erie shore to the Davis-Besse nuclear plant near Toledo, Ohio, trips.
4:11 p.m. A transmission line in northwest Ohio connecting Midway, Lemoyne and Foster substations trips.
4:11 p.m. The Perry Unit 1 nuclear reactor in Perry, Ohio, shuts down automatically after losing power.
4:11 p.m. The FitzPatrick nuclear reactor in Oswego, N.Y., shuts down automatically after losing power.
4:12 p.m. The Bruce Nuclear station in Ontario, Canada, shuts down automatically after losing power.
4:12 p.m. Rochester Gas & Electric's Ginna nuclear plant near Rochester, N.Y., shuts down automatically after losing power.
4:12 p.m. Nine Mile Point nuclear reactor near Oswego, N.Y., shuts down automatically after losing power.
4:15 p.m. FirstEnergy's Sammis-Star 345-kilovolt line, in northeast Ohio, trips and reconnects a second time.
4:16 p.m. Oyster Creek nuclear plant in Forked River, N.J., shuts down automatically because of power fluctuations on the grid.
4:17 p.m. The Enrico Fermi Nuclear plant near Detroit shuts down automatically after losing power.
4:17-4:21 p.m. Numerous power transmission lines in Michigan trip.
4:25 p.m. Indian Point nuclear power plants 2 and 3 in Buchanan, N.Y., shut down automatically after losing power.
Editor's note: this chronology is based on information gathered from the affected utilities, the North American Electric Reliability Council, the U.S. Nuclear Regulatory Commission and Genscape, a company that monitors electric transmissions.
By Jim Krane and Robert Tanner