The power is back on and people are back at their jobs on the first workday since last week's massive power outage in parts of the northeast and midwest.

In New York City, buses and subway trains were running on time, and that made Mayor Michael Bloomberg happy.

" Everything back to normal for an August Monday. Weather is good, cool, sunny, probably everything will be fine from now on," he said on NBC's Today program. "But we just have to make sure that we learn from this and try to prevent it the next time."

There were still warnings to boil water in parts of Michigan and Ohio where officials have been concerned that sewage might contaminate the water supply.

Meanwhile, average citizens and government leaders are still asking what was behind last week's power outage, the largest blackout in North American history.

"The bottom line is, if somebody gets into an elevator they want to make sure they get out," said Michigan Governor Jennifer Granholm. "If somebody turns on their tap water, they want to make sure that water comes out that is safe and the question is, who is responsible? What is the problem and how do we fix it?"

Even as the power returns, energy officials are urging consumers and business to cut back where possible to avoid rolling blackouts. Canadian officials are asking people to cut back on electricity by 50 percent if possible.

The blackout has spurred numerous calls for an upgrade of the nation's power-distribution system. But some experts also want closer regulation of the utility companies that supply the power.

Bill Richardson is the governor of New Mexico, but before that he was energy secretary in the Clinton administration.

"What has happened is faulty transmission lines caused by an overload of power, which utilities should not do and they do it around the country," he said on CBS television. "They bring excess power to deal with increased demand, they make their profits. But reliability rules declare that this is almost an illegal act, they should not do it."

The current energy secretary, Spencer Abraham, says it is too early to pinpoint the cause of last week's massive power failure. But he told CBS television it is clear that electricity rates for consumers and business will likely go up.

"Rate payers obviously will pay the bill because they are the ones who benefit," he said.

Mr. Abraham also says he hopes the blackout will convince Congress it is time to modernize the country's aging power transmission network.