Here, in question-and-answer form, is a look at what caused the worst blackout in U.S. history, why it spread so far, and what officials are trying to do about it:

Q: What happened?

A: Three transmission lines in northern Ohio tripped off, and a cascade of outages darkened the Midwest, Canada and New York, leaving millions in the dark. What caused the Ohio lines to trip remains a mystery.

Q: What are investigators looking for?

A: Any unusual surge in power in Ohio or elsewhere along the power loop circling Lake Erie. In the past, such surges have been caused by lightning strikes or faulty breakers, but investigators have more than 10,000 pages of data to examine. A detailed timeline may be available by next week.

Q: Why were power companies unable to stop the outage from spreading?

A: Investigators aren’t sure. A system of alarms is supposed to allow power operators to shut off links to other systems as they fail. The alarms were broken at FirstEnergy Corp., which owns four of the first five lines that failed. A backup alarm at the Midwest Independent Transmission System Operator, which oversees the region’s electrical grid, was in place, but a spokeswoman there said she did not know what, if anything, workers there did about the alarms.

Q: Could this happen again?

A: Yes, but experts don’t expect a repeat anytime soon.

Q: Could a similar failure happen elsewhere?

A: Probably. Federal regulators have identified at least 15 “choke points” in the power grid, where congestion can quickly overwhelm electricity transmission lines and cause blackouts. The congestion was most severe in New York state and California, but other heavily congested lines were in western Pennsylvania and between Minnesota and Wisconsin, South Dakota and Missouri, Alabama and Florida, and Georgia and Florida.

Q: What’s wrong with the power grid?

A: The aging grid has been overtaxed by consumers and businesses demanding ever more power for air conditioners, computers and the like. Altogether, the average American uses nearly 12,900 kilowatt-hours of electricity a year. The world average: 2,200. At the same time, relatively few new power plants have been built to feed the demand, and there has been relatively little investment to upgrade the power grid.

Q: Why hasn’t the system been upgraded?

A: Larry Brown, an official with Edison Electric Institute who advises the government on electric grid vulnerabilities, cites three reasons: the cost, environmental opposition and the unwillingness of communities to locate new facilities near homes.

Q: Does deregulation have anything to do with it?

A: Depends who you ask. In deregulated markets, consumers can purchase power from far-off power suppliers. That puts more stress on the transmission grids.

Critics of deregulation say that with a focus on making profits, there is less incentive to invest in new transmission lines or other infrastructure, and that national standards need to be imposed.

However, Nora Brownell, a commissioner with the Federal Energy Regulatory Commission, said the industry’s move toward competition should not be blamed. “It’s very clear this is not about deregulation. It’s about investing in the transmission system,” she said.

Q: What do experts say is needed to modernize the system?

A: FERC concluded in 2001 that it would cost about $12.6 billion to relieve congestion from the bottlenecks in the nation’s power grid. However, the report said the improvements would quickly pay for themselves in energy savings.

Q: What is the government going to do?

A: President Bush called the blackout a “wake-up call” that the power grid needs to be modernized, and formed a joint task force with Canada to review how to prevent future crises. Bush said the energy bill he has tried to push through Congress has provisions that would help address the problem. Different versions of that legislation are being negotiated by the House and the Senate.

Democrats, meanwhile, may seize on the issue in the upcoming presidential campaign – blaming Bush’s energy policies for the crisis. Massachusetts Sen. John Kerry told labor leaders the Bush administration “ignored the investment needs of our infrastructure in favor of a tax cut for the wealthy.”

AP-ES-08-17-03 1808EDT