The Bush administration said Thursday it was investigating a possible threat against the giant Palo Verde power plant in Arizona, the nation's largest nuclear plant.
The plant, near Phoenix, generates electricity for the entire Southwestern grid serving California, Nevada and Arizona.
"This is again a coordinated effort among federal agencies with the great support of the state and locals that we're addressing a very specific piece of threat information," Ridge told reporters.
Arizona Gov. Janet Napolitano said she dispatched National Guard troops to the Palo Verde plant to augment security.
"We are not in a position to comment on any specific threats that may have been made against the power plant," she said in a statement.
The probe began about 48 hours ago, Ridge said. He declined to discuss any specific information that led officials to think there might be a threat.
"It was obviously serious enough and deemed to be credible enough that we got the appropriate federal agencies investigating based on the threat information," he added.
U.S. officials were looking for Iraqi government "sleeper cells" that might carry out an attack against the Palo Verde plant, the Washington Times reported on Thursday. The newspaper cited unnamed sources who said the threat to the plant was included in classified intelligence reports.
On Monday, before U.S. troops began attacking Iraq, the U.S. government put the nation on the second-highest level of security alert. The color-coded terrorism alert level is now orange, meaning there is a high possibility of a terrorist attack.
All of the nation's 103 nuclear power plants have been on heightened alert since the Sept. 11 attacks on the World Trade Center and the Pentagon.