Terror Alert Lowered

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"Based on a careful review of the available intelligence, we have lowered the threat level to yellow," announced Homeland Security Secretary Tom Ridge Friday afternoon.

The decision to dial back the national threat level is permission for much of the country to stand down from the heightened state of alert that went into effect just before the holidays.

The extra security required of law enforcement at the local level is difficult to maintain over long periods of time and it is costly. That is one of the factors leading to the decision for most of the nation to revert back to yellow, but Ridge said, for some regions and industries the status will not change.

"There will be certain locales and certain areas within the private sector that, for the time being, we will maintain added vigilance and security," Ridge said.

Although Secretary Ridge did not specify which locales and areas, ABC News has learned airports and airlines will be among those remaining on high alert.

Security along railways and at rail stations will also remain tight. At power plants, nuclear facilities and chemical plants, officials will continue to closely monitor all security systems, and certain cities considered to be attractive targets to terrorists will also remain on elevated alert.

Those include New York, Washington, Las Vegas and Los Angeles.

Officials say they believe their vigilance over the past three weeks has paid off. They gave credit to the increased scrutiny at airports and other locations with disrupting terrorist activity.