About 40,000 customers in Manoa, McCully, Kaimuki, Waikiki, Palolo, St. Louis Heights, Mo'ili'ili, Diamond Head, Kapahulu and parts of Kahala lost power at 7:42 a.m., said Hawaiian Electric Co. spokesman Peter Rosegg.

Service was restored to some of those customers by 8:42 a.m., with the rest restored by 9:13 a.m., Rosegg said.

The cause of the failure was not known.

The line that failed — a 138-kilovolt transmission line from Kane'ohe to Palolo — is one of two that cross the Ko'olau Range, Rosegg said.

It belongs to a network of power that HECO sought to bolster with a third line until public pressure forced the company in 2002 to seek an alternate route underground. Community and environmental groups spoke out against the $31 million project, saying it was not needed and would scar a historic, undeveloped area.

"This is the scenario we feared," Rosegg said today. "We had one line out for maintenance and we lost the other one."

HECO crews have been working on structures on the Ko'olaus since January. They had shut off power to one of the 138-kilovolt lines for the job, but when the failure occurred, the crews restored it to service.

Rosegg said an inspection team will fly in a helicopter over the line to try and find the source of the failure.

There is no indication that wind was a factor, but the power lines travel through remote places prone to heavy rain and gusty winds not always felt in more populated areas, Rosegg said.

"We will move to deal with it as quickly as we can," he said.

The National Weather Service was not reporting especially strong winds this morning, with most breezes registering between 6 and 13 mph, said lead forecaster Robert Ballard.

"But we do know it has been locally gusty in places," he said. "There are places near the mountains that have gusted higher, but none of our observation posts are showing much."