US Energy Secretary Spencer Abraham told members of the Senate Armed Services Committee about the possible threat against the Palo Verde power plant in Arizona.

Homeland Security Secretary Tom Ridge told reporters that investigators from several federal agencies and state and local police were working on the reports, adding that the probe began about 48 hours ago.

Security has been tightened around the plant which generates electricity for the entire Southwestern grid serving California, Nevada and Arizona, local media reports said.

The Department of Homeland Security on Monday night raised the national terror alert to the second-highest level minutes after President George W. Bush gave Iraqi President Saddam Hussein a final ultimatum to go into exile or face a war.

The department called for increased security measures around borders, airports, nuclear power plants and over the nation's food supply around the country. It also urged governors to deploy National Guard troops or extra state police to protect key public facilities.

Officials have feared that a US nuclear power plant could be a target of sabotage or attack in response to the US military action,which began Wednesday night. Security has also been stepped up around nuclear power stations in other states such as Maryland, Alabama and Ohio.

(Xinhua News Agency March 21, 2003)