SALT LAKE CITY (AP) _ A second blackout plunged parts of Idaho into darkness on Wednesday as officials focused on a Wyoming power plant and its transmission lines as the possible source for the outage.

A short-circuit along the same transmission lines may have triggered the rapid chain-reaction outages that knocked out lights and telephone service for 1.5 million to 2 million customers in eight Western states on Tuesday.

After Wednesday's disturbance, utility officials could not re-energize the power lines from the Jim Bridger coal-fired plant near Rock Springs, Wyo., and a substation in southeastern Idaho, said Gary Donnelly, a spokesman for PacifiCorp, co-owner of the plant along with Idaho Power Co.

The utilities sent up a helicopter to inspect the 200-mile stretch of power line in hopes of discovering the problem, Donnelly said.

Wednesday's disruption apparently was limited to Idaho. Power flickered out in southern Idaho at 2:10 p.m. MDT, and the Idaho Statehouse was among the buildings that lost power for a few minutes.

``We have manually overridden the system and we are manually beginning to recover the system,'' Dennis Lopez, a spokesman for Idaho Power Co., said late Wednesday afternoon.

Utility officials said it could take a week to find out just what caused Tuesday's short-circuit, which sent disturbances rippling through the Western power grid.

``We can rule out sabotage. We can rule out UFOs. I think we can rule out computer hackers,'' said Perry Gruber, spokesman for the Bonneville Power Administration in Portland, Ore., which operates a series of Northwest hydroelectric plants.

Authorities trying to reconstruct Tuesday's events said this much is clear: Sometime shortly before 2:30 p.m. MDT, a significant disturbance rippled through the Western power grid, knocking out lights and telephone service for customers from Canada to the Southwest as computerized safeguards struggled to minimize its effect.

Within a second or two, air conditioners whirred to a stop on a day when record or near-record temperatures scorched the West. Stoplights went dark. Hospitals and air traffic control centers switched to generators. Bells and buzzers on slot-machines in Reno, Nev., briefly fell silent.

If nothing else, the short-circuit Tuesday was ``an initiating event,'' said Robert Dintelman, assistant executive director of the Western Systems Coordinating Council in Salt Lake City.

Donnelly said that by itself, ``this line tripping out yesterday should not have caused the widespread blackout. It's likely other events contributed, but we don't know what they were.''

The effects of Tuesday's outage were still being felt in Denver, where the city has to resynchronize 1,200 traffic signals, a job that will take 16 engineers several weeks, costing up to $5,000, said Amy Bourgeron, spokeswoman for Denver's Public Works Department.

Dintelman said the coordinating council, which oversees electrical service reliability in 14 Western states, two Canadian provinces and part of northern Mexico, will conduct a ``postmortem'' on the outage.

That could be difficult, he said, because investigators must review the operating charts of 76 interconnected power-generating entities comprising the council, looking for the initial glitch.

Each system monitors its power in cycles, 60 times a second. Charts of those measurements should show the sequence of the outage.

Hardest hit was Idaho, where 700,000 customers lost power.

``It was a virtual blackout,'' Dintelman said.

Some power company officials, like Donnelly, lauded the system for ``working just the way it was supposed to'' by protecting its circuits and generating plants.

But Dintelman wasn't so sure.

``If things were operating the way they were supposed to,'' he said, ``then the lights would have stayed on.''