The alert, sent by the Nuclear Regulatory Commission, said "the attack was already planned" and three people "already on the ground" were trying to recruit non-Arabs to take part, a government source said.
However CBS News has learned that everything the government knows about this latest threat comes from a single source and is apparently two or more months old.
The information came to light again because it was picked up by a foreign government and relayed to the U.S. as new information.
The source spoke of "an attack on a nuclear power plant" by "a hijacked commercial plane" and "planned by three terrorists already on the ground."
CBS News Correspondent Jim Stewart reports that from listening closely to U.S. officials, you get the impression they don't necessarily believe the threat is credible, but have no choice but to treat it as real.
"We do not discuss the details of any of our (security) advisories," said NRC spokesman William Beecher. He said that since the Sept. 11 attacks more than 20 advisories have been sent involving potential threats.
The alert stemmed from information obtained during questioning of an "al-Qaida senior operative," prompting the FBI to issue a warning to its field offices, said one government source, who spoke on condition of not being identified further.
On Jan. 23, the NRC followed up with an advisory to operators of all 103 commercial nuclear reactors at 63 sites across the country, emphasizing that the information had not been corroborated or otherwise authenticated.
The advisory said the al-Qaida operative had told the FBI "there would be a second airline attack" in the United States and "the plan is to fly a commercial aircraft into a nuclear power plant."
The FBI had been told that "three individuals were on the ground ... recruiting non-Arabs to take part in the attack" and they would choose the reactor to be targeted, the advisory said.
"The plan would include diverting the mission to any tall building if a military aircraft intercepts the plane," said the advisory, according to a source familiar it.
Security at nuclear power plants was increased immediately after the New York and Washington attacks, although federal and industry officials have acknowledged that the threat of an attack using a commercial airliner has never been specifically addressed.
The Jan. 23 advisory heightened industry awareness, but did not prompt any dramatic, new security measures.
Beecher said the nation's nuclear power plants have been "on the highest level of alert" since the Sept. 11 attacks and that has not changed.
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