The government put nuclear power plants on high alert last week, acting on a tip from an Al Qaeda operative that terrorists may be planning an airplane attack on a power reactor, government officials said Thursday.

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.

Meanwhile, the FBI's National Infrastructure Protection Center issued a "high-importance" bulletin warning about potential terrorist threats to American water-supply systems.

The alert was issued after a computer owned by an individual indirectly linked to Usama bin Laden was found to contain several software programs used for structural engineering of "dams and other water-retaining structures."

The NRC's warning to nuclear power plants stemmed from information obtained during questioning of an "Al Qaeda 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.

"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.

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 Qaeda 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.

FBI Director Robert Mueller on Thursday said there had been no new terrorist threats or warnings in the last few days.

On Wednesday, U.S. officials released an unclassified report that said they had uncovered rudimentary diagrams of nuclear weapons in a suspected Al Qaeda safehouse in Kabul, providing further evidence of Al Qaeda's efforts to acquire such weapons to use in terrorist attacks.

The unclassified report, submitted by CIA Director George Tenet to Congress, said that the "diagrams, while crude, describe essential components — uranium and high explosives — common to nuclear weapons."

The report added that the terrorists aren't believed to have a functional weapon.

Other evidence obtained in Afghanistan shows that Al Qaeda operatives have fallen for a number of scams in their attempts to acquire nuclear weapons and other weapons of mass destruction, a senior government analyst said.

Much of the terrorists' interest centers on chemical weapons, such as cyanide salts, that could be used to contaminate food and water supplies and assassinate individuals, the Tenet report says.

Further evidence uncovered from sites in Afghanistan includes diagrams of American nuclear power plants showing Al Qaeda's interest in striking these targets, but it's unclear how far along those plans were, a defense official said.

Fox News' Carl Cameron and The Associated Press contributed to this report.