nvestigators of North America's biggest blackout say all signs from a nearly completed timeline point to human errors in the early stages in Ohio on Aug. 14 as the cause of the cascade into darkness.

They have nearly finished assembling a second-by-second chronology composed of millions of bits of data collected from computers, voice recorders and hundreds of sensors scattered from Detroit through Canada into New York, officials said.

The retracing of the 600-mile electrical storm track starts at 1 p.m. on Aug. 14. Three hours passed before local problems in the Midwest grew into a crisis that cost billions of dollars and darkened the homes of millions of people.

Industry officials involved in the inquiry said they were not prepared to point to a particular cause, human or technological, but they generally voiced enthusiasm for the pace and progress of the analysis.

"We think we have the timeline nailed pretty well," said Donald M. Benjamin, vice president of the North American Electric Reliability Council, the industry group created after the 1965 blackout to maintain electricity flows.

"It's down to the second in terms of what happens, which transmission paths opened, when areas became isolated," Mr. Benjamin said. "It provides a good understanding of how the power flows."

But an expert from the federal government taking part in the investigation was much more definitive about a probable cause, saying all the data pointed to mistakes by people in the event's earliest stages.

The crucial missteps, a federal investigator working on the analysis said last night, appear to have occurred in the handling of an hourlong sequence of line failures and plant shutdowns preceding the full-blown blackout, which swept parts of eight states and eastern Canada starting around 4:10 p.m. on Aug. 14.

"Had all of the existing policies been followed, this would not have developed into a cascading event," the investigator said. "What we see are institutional breakdowns, not a breakdown of the system itself."

He and other investigators declined to discuss details, but others involved in the investigation said the timeline essentially matched independent analyses done recently by several grid experts and utilities.

The chronology also shows that by the time the problems left the Midwest, the disruption could not be stopped from exploding through the large portals linking that region with Canada and then with New York.

The reliability council, also called NERC, assembled the record for its own investigation and for a task force created by the Department of Energy and the Canadian Ministry of Natural Resources.

The findings so far will be discussed today with Spencer Abraham, the secretary of energy.

Mr. Benjamin said utilities were still assembling records from earlier in the morning of Aug. 14, with the goal of comprehending what conditions existed around the electrical grid of wires and plants before there were any signs of trouble.

Officials at the FirstEnergy Corporation, the Ohio utility whose territory and lines have been identified by many experts as the most likely trigger for the event, yesterday stood by the company's contention that there were power plant and line failures outside of its territory in the hours before its own troubles began.

"As far back as noon, there were other generation-unit trips and other transmission-line trips outside of our area," said Ralph DiNicola, a spokesman for FirstEnergy. "We're certainly hopeful that the Department of Energy and NERC are looking at all of those conditions."

Yesterday, officials from the Midwest Independent Transmission System Operator, the group responsible for overseeing the safe flow of electricity around the Midwest, said they remained convinced that the group had not contributed to the cascade.

In the Northeast, officials said they were still unable to answer some basic questions about how and why the blackout spread from the Midwest and Ontario into New York, both across the major web of transmission lines that cross the international border at the Niagara River, and at a bottleneck in upstate New York that separates the eastern part of the state from the western part.

In both places there are relays — essentially large versions of the circuit-breakers in a household fuse box — along the lines. When something goes wrong, the relays are supposed to trip, interrupting the flow of current, protecting other equipment from damage and keeping the problem from spreading.

Yet officials say many relays — which are owned by the New York State Power Authority and Niagara Mohawk, a major upstate utility — continued to conduct power even as the system gyrated out of control.